It's my opinion that this question: How can I make apt-get wait for another instance to finish? should not have been duped to How to make a package manager wait if another instance of APT is running?.
The first question had better answers (3 useful answers vs 1 useful and 2 negative score answers). It also covered the 1 useful answer from the second question slightly better.
So, I'd like for the second question to be closed as a duplicate of the first, even though it was asked first, the first is a better question.
The reason being is that I'd like to add another answer which extends upon Radu's to the question where his was posted. If my recommendation above doesn't happen, I'd be unsure about whether to post it in the second one (it seems out of place, any suggestions?).