Learn English – An expression for not giving up when you know you should

The first that comes to mind is

Birds of a feather flock together

This means that similar items (including people) are drawn to one another.

To say that "What you get is exactly what you deserve" you could use

You reap what you sow

In (British) English, a painlessly idiomatic usage would be

Mechanic: Do you want to repair your car?

You: Yes, but please don’t go overboard.

As thefreedictionary points out, ‘go overboard’ literally means falling out of a boat, but idiomatically one can also go overboard on a task: ‘to do too much; to be extravagant’.

That can mean a range of things, from taking a task in general more seriously than it could possibly merit, to polishing thousands of tiny details that no-one could possibly ever see.

In the case of your example, the not going overboard scenario would involve the mechanic simply fixing the essential problem efficiently, and stopping there.

John’s colleagues would understand each other perfectly if one of them were simply to say, ‘John is going overboard again.’ They might or might not try to stop him, but they would understand that John is expending unnecessary time and effort, as usual.