At the moment in SourceTree for Windows v3.4.17,
Checking-out a git branch that already exist locally, shows an ugly red error that says:

_{git -c diff.mnemonicprefix=false -c core.quotepath=false --no-optional-locks checkout -b master --track origin/master
fatal: a branch named 'master' already exists
Completed with errors, see above.}

Please, instead of showing that error, show another dialog to allow the user to "checkout the existing local branch and pull changes",
just like GitGraph tool for VS-Code does:

This is a task that developers have to do everyday, to change from one branch to the "master" branch by clicking in the branch label shown in the Graph.