Not sure what you mean by a duplicate source - do you mean, e.g., that the 1910 census is listed twice? I don't think there is a way to merge your source citations, but why not just delete one of them? If they are actually supporting different facts/events, the answer will be in the same process used to see what facts are connected to the source (below).
To see what facts are connected to a source, first click on the facts/sources tab, above the timeline. Then click on "source citations" on the next screen. To see what facts are associated with a particular source, or to have that source support additional facts (including ones you just added to the timeline, based on something that is in the source), click on the link next to the word "Detail." On the next screen, at the right side of the page, there is a boxed area where there are two links, "edit citation" and "delete citation." If you click on "edit citation" and go down to the bottom of that screen, you will see all timeline facts for which that source is cited. You can check or uncheck anything in the timeline, to add or remove that source as authority for a particular fact.
O:ne thing about the "edit" screen that can cause problems: you MUST put something (literally anything, even a period) in the box for "detail," or the process will stall and not complete.
Hope this helps.