Pull Request has already been merged

We are using workzone plugin for codereviews anad automerge. While auto merge, the page is getting refreshed which somehow triggers the Pull request merge(again via api) "Pull Request has already been merged" error pops up in the UI.

Pasted the stack trace, from the error it looks like its not Workzone plugin issue. Can you please look into this.

2016-06-15 16:56:52,453 DEBUG [pool-5-thread-1] c.a.bitbucket.scm.BaseCommand Executed /etrade/sit-git-app-bitbucket/packages/git/bin/git log 971070bb916a3462beeaa72ec11a1b6ac68ee47f..2872275be5817d4c00e2873184627b41d94e01c3 --pretty=format:%H --shortstat --no-merges
2016-06-15 16:56:52,453 WARN [pool-5-thread-1] c.s.b.g.idx.commits.ao.AoGraphsDao ((Diffs)) -> 14
2016-06-15 16:56:52,457 WARN [pool-5-thread-1] c.s.b.g.idx.commits.ao.AoGraphsDao --> 4:
2016-06-15 16:56:52,548 DEBUG [http-bio-8443-exec-13] kraju1 @1VGAGP4x1016x1813x0 ghwyr0 10.21.81.213 "GET /rest/api/latest/projects/GIT/repos/gitflowfork/pull-requests/18/merge HTTP/1.1" c.a.s.i.r.e.ServiceExceptionMapper Mapping ServiceException to REST response 409
com.atlassian.bitbucket.pull.IllegalPullRequestStateException: This pull request has already been merged.
at com.atlassian.stash.internal.pull.DefaultPullRequestService.checkIsOpen(DefaultPullRequestService.java:1050) ~[bitbucket-service-impl-4.5.1.jar:na]
at com.atlassian.stash.internal.pull.DefaultPullRequestService.canMerge(DefaultPullRequestService.java:267) ~[bitbucket-service-impl-4.5.1.jar:na]
at com.atlassian.plugin.util.ContextClassLoaderSettingInvocationHandler.invoke(ContextClassLoaderSettingInvocationHandler.java:26) ~[atlassian-plugins-core-4.1.1.jar:na]
at org.eclipse.gemini.blueprint.service.importer.support.internal.aop.ServiceInvoker.doInvoke(ServiceInvoker.java:56) ~[na:na]
at org.eclipse.gemini.blueprint.service.importer.support.internal.aop.ServiceInvoker.invoke(ServiceInvoker.java:60) ~[na:na]
at org.eclipse.gemini.blueprint.service.util.internal.aop.ServiceTCCLInterceptor.invokeUnprivileged(ServiceTCCLInterceptor.java:70) ~[na:na]
at org.eclipse.gemini.blueprint.service.util.internal.aop.ServiceTCCLInterceptor.invoke(ServiceTCCLInterceptor.java:53) ~[na:na]
at org.eclipse.gemini.blueprint.service.importer.support.LocalBundleContextAdvice.invoke(LocalBundleContextAdvice.java:57) ~[na:na]
at com.atlassian.stash.internal.rest.pull.PullRequestResource.canMerge(PullRequestResource.java:427) ~[bitbucket-rest-4.5.1.jar:na]
at com.atlassian.applinks.core.rest.context.ContextFilter.doFilter(ContextFilter.java:26) [applinks-plugin-5.0.5_1459832966000.jar:na]
at com.atlassian.applinks.core.rest.context.ContextFilter.doFilter(ContextFilter.java:26) [applinks-plugin-5.0.5_1459832966000.jar:na]
at com.atlassian.applinks.core.rest.context.ContextFilter.doFilter(ContextFilter.java:26) [applinks-plugin-5.0.5_1459832966000.jar:na]
at com.atlassian.applinks.core.rest.context.ContextFilter.doFilter(ContextFilter.java:26) [applinks-plugin-5.0.5_1459832966000.jar:na]
at com.atlassian.applinks.core.rest.context.ContextFilter.doFilter(ContextFilter.java:26) [applinks-plugin-5.0.5_1459832966000.jar:na]
at com.atlassian.plugin.connect.plugin.auth.scope.ApiScopingFilter.doFilter(ApiScopingFilter.java:89) [atlassian-connect-plugin-1.1.70-bitbucket-01.jar:na]
at com.atlassian.stash.internal.spring.security.StashAuthenticationFilter.doFilter(StashAuthenticationFilter.java:88) [classes/:na]
at com.atlassian.stash.internal.web.auth.BeforeLoginPluginAuthenticationFilter.doInsideSpringSecurityChain(BeforeLoginPluginAuthenticationFilter.java:109) [classes/:na]
at com.atlassian.stash.internal.web.auth.BeforeLoginPluginAuthenticationFilter.doFilter(BeforeLoginPluginAuthenticationFilter.java:75) [classes/:na]
at com.atlassian.security.auth.trustedapps.filter.TrustedApplicationsFilter.doFilter(TrustedApplicationsFilter.java:94) [atlassian-trusted-apps-core-4.2.0.jar:na]
at com.atlassian.oauth.serviceprovider.internal.servlet.OAuthFilter.doFilter(OAuthFilter.java:67) [atlassian-oauth-service-provider-plugin-2.0.3_1459800554000.jar:na]
at com.atlassian.core.filters.ServletContextThreadLocalFilter.doFilter(ServletContextThreadLocalFilter.java:21) [atlassian-core-4.6.19.jar:na]
at com.atlassian.core.filters.AbstractHttpFilter.doFilter(AbstractHttpFilter.java:31) [atlassian-core-4.6.19.jar:na]
at com.atlassian.plugin.connect.plugin.auth.user.ThreeLeggedAuthFilter.doFilter(ThreeLeggedAuthFilter.java:122) [atlassian-connect-plugin-1.1.70-bitbucket-01.jar:na]
at com.atlassian.plugin.connect.plugin.auth.oauth.OAuth2LOFilter.doFilter(OAuth2LOFilter.java:82) [atlassian-connect-plugin-1.1.70-bitbucket-01.jar:na]
at com.atlassian.jwt.internal.servlet.JwtAuthFilter.doFilter(JwtAuthFilter.java:32) [jwt-plugin-1.5.5_1459832966000.jar:na]
at com.atlassian.analytics.client.filter.DefaultAnalyticsFilter.doFilter(DefaultAnalyticsFilter.java:38) [analytics-client-5.2.1_1459832966000.jar:na]
at com.atlassian.analytics.client.filter.AbstractHttpFilter.doFilter(AbstractHttpFilter.java:39) [analytics-client-5.2.1_1459832966000.jar:na]
at com.atlassian.stash.internal.web.auth.BeforeLoginPluginAuthenticationFilter.doBeforeBeforeLoginFilters(BeforeLoginPluginAuthenticationFilter.java:87) [classes/:na]
at com.atlassian.stash.internal.web.auth.BeforeLoginPluginAuthenticationFilter.doFilter(BeforeLoginPluginAuthenticationFilter.java:73) [classes/:na]
at com.atlassian.stash.internal.request.DefaultRequestManager.doAsRequest(DefaultRequestManager.java:86) [bitbucket-service-impl-4.5.1.jar:na]
at com.atlassian.stash.internal.hazelcast.ConfigurableWebFilter.doFilter(ConfigurableWebFilter.java:38) [classes/:na]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_40]
at java.lang.Thread.run(Thread.java:745) [na:1.8.0_40]
... 279 frames trimmed
2016-06-15 16:56:52,667 DEBUG [http-bio-8443-exec-13] kraju1 @1VGAGP4x1016x1821x2 ghwyr0 10.21.81.213 "GET /rest/jira/latest/projects/GIT/repos/gitflowfork/pull-requests/18/issues HTTP/1.1" c.a.b.i.i.j.DefaultJiraIssueService Bitbucket has not been linked to JIRA
2016-06-15 16:56:52,699 DEBUG [http-bio-8443-exec-19] kraju1 @1VGAGP4x1016x1823x1 ghwyr0 10.21.81.213 "GET /rest/api/latest/inbox/pull-requests/count HTTP/1.1" c.a.s.i.rest.inbox.InboxResource Retrieving pull request count for user