Try restarting Thunderbird in safe mode, with add-ons disabled. (Click on the three-bar symbol to the right of the search bar, select Help, and then Restart with Add-ons Disabled.)

The email addresses are still doubled.

You say "the first entry in each line" - does this mean that you're putting multiple addresses on a line?

Thunderbird prefers one address per line. It allows multiple addresses, separated by commas, but you lose the ability to identify unknown or mistyped addresses.

I think if you enter multiple addresses and then press return, it will break them apart into one-address-per-line format.

Can you advise whether you are using one address per line, or multiple addresses per line? If multiple, does return break them apart? If so, does it help with the doubling up?

The doubling up has to be a bug, but it would be useful to know how to provoke it.

I have been putting multiple addresses with each being separated by a comma. If I enter each address separately, the email address appears twice without a comma and cannot be sent until one of the addresses is deleted.

Thus, if I send a message to mmillrdoc@yahoo.com, the address is entered as mmillrdoc@yahoo.commmillrdoc@yahoo.com on the email.

And if you try one entry per line?

Unless you're copy and pasting from a list managed elsewhere, I can't see any benefit in comma-separated addresses. You have to type them in longhand, or copy and paste very carefully.

Try return and putting each address on its own line.

Then each entry is doubled as above.

Which version of Thunderbird is this? What platform do you use? Windows, Mac, Linux?

Do your address book entries have appropriate entries in all the relevant fields? First name, Last name, Display name, Nickname, email address?

Thank you for your suggestion

I am running Thunderbird 45.0 on a iMac. The change in the address book did not solve the issue.