Switch from T-Mobile to Xfinity Issues

Despite hours spent at the Xfinity store in Arlington, VA having my 3 lines ported from T-Mobile to Xfinity,

I later found out T-Mobile had continued to charge me for 3 more months after the switch for 2 lines which they claim were still active (never used them-didn't know they existed)  Why didn't Xfinity switch my lines over and close the T-Mobile account? They literally spent hours on the phone with T-Mobile over 2 days in the Xfinity store trying to make the switch. I am out over $500 and many hours wasted trying to fight this with T-Mobile who blame Xfinity. I would like some reimbursement!