Help...when importing data many records are skipped, even with a unique identifier.

Help...when importing data many records are skipped, even with a unique identifier.

I'm trying to import about 740 records.  Each time I import I only get 221.  It says that 223 are skipped and I'm not sure why.  Some of the records have duplicate email addresses, hence why I created a client ID field.  How do I make the Client ID field the unique identifier and not email?