You can't merge facts in RM. The best you can do with this situation is to delete one of the marriage facts. If they are identical, then it doesn't matter which one you delete. If they are not identical (for example, if one has more sources than the other), you will have to decide which one to keep and which one to delete. You might want to manually enter any data from the one your are going to delete into the one you are going to keep if the one you are going to keep doesn't already have the data.
I'm assuming that the duplicate marriages are for the same date. If they are not for the same date, you may want to keep both of them. I have quite a few occasions where a couple was married, divorced, and remarried. In one case they were divorced and remarried twice. If this is your situation, then you will want to keep all the marriage facts.
If the duplicate marriages really are for the same date, there is one more thing to worry about. As you work on the two marriage facts, they may go all jumpy around on you. By that, I mean that the two marriage facts might change order on you without you realizing that it has happened. This is a very bad habit that RM has - changing the order of same date facts. So if you decide the delete the first marriage fact and keep the second marriage fact on the same date, by the time you do the delete you could easily be deleting the wrong one by accident. If I encountered this situation, I would give the two marriage different sort dates so they wouldn't go all jumpy around on me.