Fusion is a concept in Star Trek Timelines that applies to crew members. Because of the temporal anomaly, there can be multiple versions of each character running around, and if two of the crew at the same point meet, they can be fused. Fusing characters increases their powers (see Crew Progression for detail). Characters with all their stars filled in cannot be fused further, and any more duplicates can be dismissed.
If the player accidentally fuses their characters so that they have two four star Captain Janeways with 2 out of 4 stars filled, and they fuse those two Captain Janeways, they will only have a Captain Janeway with three stars filled, not four.
Honorable Citations can be used to add another star to a crew member without having to find a duplicate to fuse with.
See also: Fusion of Different Characters