What causes duplicate or alias data in CRMs?

Duplicate data appears when the same subscriber enters your system multiple times through variations of the same email. Some providers support plus addressing such as captain+promo@tidalmail.com which still delivers to captain@tidalmail.com. Gmail also supports dot aliases meaning captain@gmail.com and c.a.p.t.a.i.n@gmail.com deliver to the same inbox. These variations look unique in a CRM but distort engagement metrics and segmentation.