Steps to replicate
Login as "User A"
1. Create a new space "Foo".
2. Create another space "Bar"
3. In the Foo space create a page "Foo".
4. In the Bar space create a page "Bar"
5. In both spaces create a page under the project page named "foobar" (same name in both spaces, different content)
Page from Foo

Page from Bar

6. Restrict the access to page "foobar" from Space 'Foo" to User A only.

Login as "User B"
7. Because of the page restriction, the "foobar" page from Space Foo is not visible/accessible.
8. Open Bar page in space Bar.
9. Move Bar page (and sub page) to the space Foo.
10. Confluence moves the Bar page and the foobar child page and fails to detect that there's already a foobar page in the Foo space.

Login as "User A"
11. The page tree of space Foo contains the page foobar twice (under "Bar" and "Foo"). Both foobar pages point to the same physical page. The other page is gone. (The page with content "From A and only A" is missing)

Possible workaround for the missing page

Move the page back to the original location, you should be able to see both page's original content.
Rename the page then only move to the desired location