Flip side of the flip side is bad because when you fix the bug in "abstraction" or de-duplicated piece of code: how do you know you did not break something you

Flip side of the flip side is bad because when you fix the bug in "abstraction" or de-duplicated piece of code: how do you know you did not break something you don't know. Duplication is easier because once you fix that single place - you are 100% sure you fixed that place and you did not break 10 other places. Maybe you know "by writing unit tests", but when you write unit tests, when you find out you broke something. Funny story time: had an add/edit popup in system because they looked the same so dev just made it "single thing". Something like 3 months dev1 fixed something -> qa2 found a bug X -> dev2 fixed something -> qa1 found a bug Y -> dev3 fixed something -> qa 3 again found bug X. When I got into code base I noticed that ping-pong because somehow I was only sane person to check git history and I split things up. Something like that happened multiple times in my career