This can happen in two ways:
1. We have a page with the following content (as example):

Page content

First line here

And the second line

2. Two different pages within a space have anchors with the same name. Since all the pages can be exported as ONE single PDF, the effect is like #1 (two similar anchors within one document)....

Anyways, here is the exception:

com.atlassian.confluence.importexport.ImportExportException: org.apache.fop.apps.FOPException: stream:44:0 The id "features" already exists in this document at com.atlassian.confluence.importexport.impl.PdfExporter.generatePDF(PdfExporter.java:210) at com.atlassian.confluence.importexport.impl.PdfExporter.doExportEntity(PdfExporter.java:187) at com.atlassian.confluence.importexport.impl.AbstractRendererExporterImpl.exportPage(AbstractRendererExporterImpl.java:147) at com.atlassian.confluence.importexport.impl.AbstractRendererExporterImpl.doExport(AbstractRendererExporterImpl.java:113) at com.atlassian.confluence.importexport.impl.PdfExporter.doExport(PdfExporter.java:82) at com.atlassian.confluence.importexport.DefaultImportExportManager.exportAs(DefaultImportExportManager.java:76) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:324) at org.springframework.aop.framework.AopProxyUtils.invokeJoinpointUsingReflection(AopProxyUtils.java:59) at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:201) at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:176) at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:153) at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:196) at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:135) at $Proxy21.exportAs(Unknown Source) at com.atlassian.confluence.importexport.actions.ExportPageAction.execute(ExportPageAction.java:74) at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:173) at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:37) at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:170) at com.opensymphony.xwork.interceptor.DefaultWorkflowInterceptor.intercept(DefaultWorkflowInterceptor.java:58) at com.atlassian.confluence.core.ConfluenceWorkflowInterceptor.intercept(ConfluenceWorkflowInterceptor.java:32) at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:170) at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:37) at com.atlassian.confluence.core.ConfluenceValidationInterceptor.intercept(ConfluenceValidationInterceptor.java:18) at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:170) at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:37) at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:170) at com.atlassian.confluence.security.actions.PermissionCheckInterceptor.intercept(PermissionCheckInterceptor.java:40) at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:170) at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:37) at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:170) at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:37) at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:170) at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:37) at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:170) at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:37) at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:170) at com.opensymphony.xwork.DefaultActionProxy.execute(DefaultActionProxy.java:116) at com.opensymphony.webwork.dispatcher.ServletDispatcher.serviceAction(ServletDispatcher.java:182) at com.opensymphony.webwork.dispatcher.ServletDispatcher.service(ServletDispatcher.java:162) at javax.servlet.http.HttpServlet.service(HttpServlet.java:103) at com.caucho.server.http.FilterChainServlet.doFilter(FilterChainServlet.java:96) at com.opensymphony.module.sitemesh.filter.PageFilter.parsePage(PageFilter.java:129) at com.atlassian.confluence.util.profiling.ProfilingPageFilter.parsePage(ProfilingPageFilter.java:36) at com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:61) at com.caucho.server.http.FilterChainFilter.doFilter(FilterChainFilter.java:88) at com.atlassian.seraph.filter.SecurityFilter.doFilter(SecurityFilter.java:161) at com.caucho.server.http.FilterChainFilter.doFilter(FilterChainFilter.java:88) at com.atlassian.seraph.filter.LoginFilter.doFilter(LoginFilter.java:181) at com.caucho.server.http.FilterChainFilter.doFilter(FilterChainFilter.java:88) at com.atlassian.johnson.filters.JohnsonFilter.doFilter(JohnsonFilter.java:96) at com.caucho.server.http.FilterChainFilter.doFilter(FilterChainFilter.java:88) at org.springframework.orm.hibernate.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:83) at com.atlassian.confluence.setup.SpringSessionInViewFilter.doFilterInternal(SpringSessionInViewFilter.java:32) at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:39) at com.caucho.server.http.FilterChainFilter.doFilter(FilterChainFilter.java:88) at com.atlassian.util.profiling.filters.ProfilingFilter.doFilter(ProfilingFilter.java:132) at com.caucho.server.http.FilterChainFilter.doFilter(FilterChainFilter.java:88) at com.atlassian.core.filters.AbstractEncodingFilter.doFilter(AbstractEncodingFilter.java:38) at com.caucho.server.http.FilterChainFilter.doFilter(FilterChainFilter.java:88) at com.caucho.server.http.Invocation.service(Invocation.java:315) at com.caucho.server.http.CacheInvocation.service(CacheInvocation.java:135) at com.caucho.server.http.HttpRequest.handleRequest(HttpRequest.java:246) at com.caucho.server.http.HttpRequest.handleConnection(HttpRequest.java:163) at com.caucho.server.TcpConnection.run(TcpConnection.java:139) at java.lang.Thread.run(Thread.java:534) Caused by: org.apache.fop.apps.FOPException: stream:44:0 The id "features" already exists in this document at org.apache.fop.datatypes.IDReferences.createID(IDReferences.java:119) at org.apache.fop.fo.FObjMixed.layout(FObjMixed.java:116) at org.apache.fop.fo.flow.Block.layout(Block.java:257) at org.apache.fop.fo.flow.Block.layout(Block.java:257) at org.apache.fop.fo.flow.Block.layout(Block.java:257) at org.apache.fop.fo.flow.AbstractFlow.layout(AbstractFlow.java:154) at org.apache.fop.fo.flow.AbstractFlow.layout(AbstractFlow.java:110) at org.apache.fop.fo.pagination.PageSequence.makePage(PageSequence.java:400) at org.apache.fop.fo.pagination.PageSequence.format(PageSequence.java:338) at org.apache.fop.apps.StreamRenderer.render(StreamRenderer.java:262) at org.apache.fop.fo.FOTreeBuilder.endElement(FOTreeBuilder.java:223) at com.caucho.xml.XmlParser.popNode(XmlParser.java:883) at com.caucho.xml.XmlParser.closeTag(XmlParser.java:784) at com.caucho.xml.XmlParser.parseNode(XmlParser.java:357) at com.caucho.xml.XmlParser.parseInt(XmlParser.java:246) at com.caucho.xml.AbstractParser.parse(AbstractParser.java:614) at org.apache.fop.apps.Driver.render(Driver.java:498) at org.apache.fop.apps.Driver.run(Driver.java:565) at com.atlassian.confluence.importexport.impl.PdfExporter.generatePDF(PdfExporter.java:206) ... 67 more