The "How many collections are you currently working on?" thread brought a question to mind. There are a lot of standard collections that require the same coins. For example, a 20th century type set would require several Lincolns that would also need to be present in a Lincoln series set. When this happens, what do you do? Duplicates of the same coin? Or does one example fill both slots? Personally one coin suffices; I'm so poor that even my coins have to take second jobs.
I had never given it a thought of what to do. I have simply one collection, a type set, that is my primary collection. Then, the Lincoln or Roosevelt or Washington, et cetera, series folder gets the second best (or worst depending on price).
I may buy a duplicate but not because of sets. Sometimes I just fall in love with a pretty coin and have to have it if the price is fair.
One coin definitely fills both slots. I know that the NGC Registry allows this (and I've done it a few times). I'm not going to buy 2 of the same coin just to fit it into 2 different sets. The only time I would do that would be if I were displaying both sets at the same time, at a show or something.
Duplicates all the way; I just can't abide by holes! And this is why I have triplicates of so many coins!
For me, one coin does it for all of the collections most of the time. Here is one of the few exceptions. I have a certified 1908 I find attractive and an MS-65 graded 1925-D for type. Pulling those two coins out of this set would ruin the look, I won't get much for them anyway.
Coins pull double-duty - mostly. For example, I have an 1836 half dime in MS-63 that is my type coin and also one of my Capped Bust Half Dime collection. OTOH, the SLQs in my type set are all MS-64s and 65s but my SLQ collection is all circulated coins - so no double duty there. So double-duty is the policy for economy reasons unless the collecting standards do not allow it.
