I'm banging my head against table for this issue. Actually, all of sudden my system (Windows 10) shut down. When it came on, following error in git pull arrived
git fatal: SHA1 COLLISION FOUND
I sorted out this issue as per this link git fatal: SHA1 COLLISION FOUND
Everything is working fine now except this following error on PUSH through Git GUI
error: window name "push_setup" already exists in parent