Yes, there were duplicate WBS codes from one sub-project to the other that showed up when viewing the master schedule. Since they were so numerous, I had to go into each sub-project and delete all WBS codes then re-number them. When I opened the master schedule file, the message did not appear (even though there were still duplicate WBS codes from the sub-projects). I saw in a related post that this might be an issue when converting files from Project 2010 to 2016. Thanks for your input.