After you file

Is there any problem with mailing it again?  I mailed my amended return (certified) in May.  Tracking through the post office showed it was received.  Tracking on the IRS website still shows no record.

Because there is a time limit for amending your returns (I am due a refund from 2018)  I mailed it AGAIN certified last week.  Tracking through the post office shows received.  I don't want to mess anything up by mailing it twice- but after 5 months and the IRS having no record of it- I assumed it was lost and sent it again.