Pull request from forked repo: "A branch with that name already exists."

Describe the bug

After upgrading to 2.6.1 (apparently), re-selecting a PR branch from a forked repo fails with the error “A branch with that name already exists.”

Version & OS

• GH Desktop: 2.6.1
• MacOS: 10.14.6

Steps to reproduce the behavior

1. Clone the storybook repo: https://github.com/storybookjs/storybook
2. Select a PR from a forked branch, e.g. pr/13142
3. Select a non-forked branch, e.g. next
4. Re-select the PR from the forked branch in #2
5. See error “A branch with that name already exists.”

Note that selecting a second PR from the same fork, e.g. pr/13429, will succeed after performing these steps. And then selecting another branch and re-selecting pr/13429 will fail with the same error.

Expected behavior

I expect to be able to freely switch between branches, including PRs from forked repos.

Actual behavior

Selecting a branch on a forked repo fails the second time around.

Logs

2020-12-16T02:11:24.810Z - info: [ui] Executing fetch: git -c credential.helper= -c protocol.version=2 fetch --progress --prune github-desktop-BarryThePenguin (took 2.222s)
2020-12-16T02:11:25.178Z - error: [ui] `git branch pr/13429 github-desktop-BarryThePenguin/update-ember-cli` exited with an unexpected code: 128.
stderr:
fatal: A branch named 'pr/13429' already exists.

(The error was parsed as 26: A branch with that name already exists.)

Additional context

• Checking out PRs from forked repos is a regular part of my workflow, and the problem seems to have been introduced very recently, probably in v2.6.1.
• Manually deleting the branch locally on the command line git branch -d pr/13429 is a temporary workaround.

[jlund24]

for those with this issue, here’s how I resolved it on GitHub Desktop:

• in your forked branch (e.g. “master”), select History in the left panel
• click “select Branch to Compare…”
• select “upstream/master” or your comparable branch that has a conflict to be resolved
• this will bring up a dialog for actually resolving the conflicts

[fnehmer]

Facing the bug in Version 2.8.6 (x64) Windows aswell