If someone else sends you a piece of code to add to the program you are working on, we need legal papers to use it--the same sort of legal papers we will need to get from you. Each significant contributor to a program must sign some sort of legal papers in order for us to have clear title to the program. The main author alone is not enough.
So, before adding in any contributions from other people, tell us so we can arrange to get the papers. Then wait until we tell you that we have received the signed papers, before you actually use the contribution.
This applies both before you release the program and afterward. If you receive diffs to fix a bug, and they make significant change, we need legal papers for it.
You don't need papers for changes of a few lines here or there, since they are not significant for copyright purposes. Also, you don't need papers if all you get from the suggestion is some ideas, not actual code which you use. For example, if you write a different solution to the problem, you don't need to get papers.
I know this is frustrating; it's frustrating for us as well. But if you don't wait, you are going out on a limb--for example, what if the contributor's employer won't sign a disclaimer? You might have to take that code out again!
The very worst thing is if you forget to tell us about the other contributor. We could be very embarrassed in court some day as a result.