if you have two questions about difference things but they have the same answer.
- Should the answer be linked form one to another?
- Should it be copy-pasted?
Are there some strict rules on this subject?
If you feed the system three or more copypastas, it'll be sick and tell the moderators on you. I don't say that to say it's wrong but it's there to stop people plastering the site in junk.
First really ask yourself if one of the questions needs to be open. If copy-and-pasting works completely, there's an expectation that one of the questions will probably fit inside the scope of the other one.
I know that's not always the case, but that's the barrier you need to overcome.
After that, do what makes most sense in the situation. Is a duplicate a good thing or a bad thing?
I have very rarely seen any need to copy-paste one of my previous answers onto a new question, but there have been occasions, and on those occasions:
If it's a significantly long answer, I'll often say at the top "Please permit me to copy-paste from one of my [other answers](link):" I don't feel obliged to do this, I do this just as a courtesy and to reassure anyone who might think I'm trying to pull a fast one.
Often the question will be slightly different to the other one, so I'll edit (or totally rewrite) the copy-pasted answer anyway, just to make it more relevant.
I only do it with my own anwsers, not other people's. If I need to refer to someone else's answer from another question, I'll do what you would typically do when linking to anywhere: include a link and a relevant, but not substantial, quote from the original.