SourceTree may not allow the user to open a local repository, and will instead select another tab that is already open. This happens when the repo is also a submodule of another repo and the submodule is already open in another tab. Steps to reproduce:

Git repository 'A' cloned to C:\GitRoot\A

Git repository 'B' cloned to C:\Git\Root\B

Repo B is also a submodule of A, so a copy also exists in C:\GitRoot\A\B.

Open repo A.

In Submodules section of repo A, double-click repo B to open it in another tab.

Click '+' to the right of tabs. In Local Repositories double-click repository B (i.e. the clone in C:\GitRoot\B).

A new tab should open for repo B, however the repo B (submodule) tab is selected instead.

This is a fairly new bug in SourceTree that affects version 3.3.4 (which is not available in the Jira Affected Version/s field).