Experience similar to OP's on a TATL last month. I checked a bag at the airport counter and shortly afterward received the email about overlapping reservations. I phoned UA and spoke with a knowledgeable agent, and was asked for identifying information (address, phone number, birth date, plus other identifiers). The *overlapping* reservation was for a flight the following day, with different origin and destination airports, for an individual with the same first and last name as mine but with a different middle initial. My ticket (and MP account) actually had my full middle name and a suffix which the *overlapping* reservation did not. Agent put me on hold for >30 minutes for a supervisor to verify information, and then told me the reservations had been separated. I asked the agent to note this in the record in case the issue resurfaced and affected my return flight. The agent understood the problem and was helpful, but still - 30+ minutes on hold to resolve.