The first time you do it, just get the job done as described. The second time, look for ways the two might be similar, but don't spend a lot of time on it. The third time, generalize and assume there will be a fourth, fifth, sixth, etc. time.

In the example the top poster gave, the first time it was requested, I would just write up some code for triggering the notification. The second time, maybe there's a shared class (or whatever) for notifications with text passed in. The third time, now maybe we need the ability to specify notification text in config or DB or through user-triggered actions.

But customers will never tell you all their requirements up front, and it's a PM's job when hearing a need from a customer to take the time to figure out if it's a single unique stand-alone problem, or simply one instance of an entire category, the other 3687 of which your customers are going to come asking for the moment they realise it's a category of problem your product is able to help with at all.