If a file already exists in tf do a check deep
git tf checkin --deep
Checking in to $/Project: 0%, commit 8b31b9d git-tf: item 'Project/background.png' exists in commit 8b31b9d more than once with different casing. TFS does not support having the same item with different cases in the same path.
And as stated in the error TFS does not support different cases for the same file or path.