Consider this situation:

local 'develop' branch tracks a remote 'origin/develop' branch.

If I double-click the remote branch (origin/develop), I would expect to check out the local tracking branch (develop), and then for that branch to be fast-forwarded, if possible.

Instead, Sourcetree obtusely tries to checkout the branch by creating a new local branch with the same name as my existing local tracking branch.  This is clearly unexpected behavior.

Conservatively, I'd expect to be switched to the existing local tracking branch.  Or be prompted, since I already have that one.  But best behavior would be to checkout the local and then fast-forward.