Work on giving them unique traits that set them apart from the existing character, either personality wise or appearance wise, whichever you're worried about.
For physical, try changing up their build, face-shape, colour-scheme, etc. Maybe this character wears glasses and the other doesn't, or maybe they have freckles or a birthmark. They don't have to be big changes, just enough that you can easily tell them apart.
For personality stuff, give them their own motives and reasons for their behaviors, and think of some unique quirks or ticks for them.
Just change them up in small but significant ways to help define their individuality as an OC.
Characters sharing first names can be confusing at times, but it's a not a big deal; I have a one or two with the same first name myself.
Hope this helped your question!