This notification indicates that there are files with the same file name, but with different case. For example there may be a file named ‘Test.txt’ and another named ‘test.txt’.
While this may be accepted on some operating systems, it may not be on others. Since the client may synchronize data to a number of devices, this may become an issue in the future if the folder is joined by a device where duplicate names are not permitted.
When this notification appears in the client, you can select an new, unique name for the file and rename it.