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diff --git a/doc/user/project/repository/branches/index.md b/doc/user/project/repository/branches/index.md index 4e3510c49b7..60504b94502 100644 --- a/doc/user/project/repository/branches/index.md +++ b/doc/user/project/repository/branches/index.md @@ -145,6 +145,38 @@ To view the **Branch rules overview** list: ## Troubleshooting +### Multiple branches containing the same commit + +At a deeper technical level, Git branches aren't separate entities, but labels +attached to a set of commit SHAs. When GitLab determines whether or not a branch has been +merged, it checks the target branch for the existence of those commit SHAs. +This behavior can cause unexpected results when two merge requests contain the same +commits. In this example, branches `B` and `C` both start from the same commit (`3`) +on branch `A`: + +```mermaid +gitGraph + commit id:"a" + branch "branch A" + commit id:"b" + commit id:"c" type: HIGHLIGHT + branch "branch B" + commit id:"d" + checkout "branch A" + branch "branch C" + commit id:"e" + checkout main + merge "branch B" id:"merges commits b, c, d" +``` + +If you merge branch `B`, branch `A` also appears as merged (without any action from you) +because all commits from branch `A` now appear in the target branch `main`. Branch `C` +remains unmerged, because commit `5` wasn't part of branch `A` or `B`. + +Merge request `A` remains merged, even if you attempt to push new commits +to its branch. If any changes in merge request `A` remain unmerged (because they +weren't part of merge request `A`), open a new merge request for them. + ### Error: ambiguous `HEAD` branch exists In versions of Git earlier than 2.16.0, you could create a branch named `HEAD`. |