In Eberron: Rising from the Last War, the changeling race says the following (p. 18):
Ability Score Improvement. Your Charisma score increases by 2. In addition, one ability score of your choice increases by 1.
Can you choose Charisma again for your additional +1, for a total of +3 to Charisma? It seems a bit cheeky to me, since no other race can have a +3 at character creation, but on the other hand, the two sentences do seem to be rather deliberately disjointed. It's not like the wording of the half-elf's ability score improvement, as this Q&A explains, so does that mean that gaining a +3 to Charisma is a legitimate choice?
Obviously this is assuming that a DM doesn't insist otherwise, and we are also assuming that this won't allow an ability score to go above 20.