Can you e-file a return tat has already been mailed?

Can you?  Yes.  Should you?  NO.

The IRS has already received a 2019 tax return from you.  If you e-file a 2019 tax return with the exact same information as the mailed tax return, this will cause confusion with the IRS systems and delay for a substantial amount of time any federal tax return you were expecting.

Additionally, the IRS was mot processing any paper filed tax returns for several months due to the pandemic and limited staffing.  They have just recently started to process approximately 11 million paper returns in the order received.

See this IRS website for information - https://www.irs.gov/newsroom/irs-operations-during-covid-19-mission-critical-functions-continue

We’re experiencing delays in processing paper tax returns due to limited staffing. If you already filed a paper return, we will process it in the order we received it.

Do not file a second tax return or contact the IRS about the status of your return.