Am using Claws 4.1.0 on Mageia Cauldron (testing, x86_64). When replying to an email, the Compose window displays two "To:" fields (with only one filled in containing the recipient's name/address), along with a scroll bar. I have not found a way to delete the second "To:" field, nor can that section of the window be expanded to remove the appearance of the scroll bar. I would attach a screenshot, but it would expose personal (account) information.
I checked a second Linux OS I'm trying out (in a virtual machine) that also has Claws 4.1.0. With that, the Compose window on a reply, also displays two "To:" fields, but that section of the window can be expanded, eliminating the scroll bar. Would this now be an issue specific to Mageia?
That's not a bug. That is how you, optionally, add another field, be it a To:, Cc:, Bcc:, or whatever.
This sounds like something you ought to have discussed on the Claws Mail users@ mailing-list before dumping a ticket into bugzilla. About providing a screenshot, it wouldn't require much effort to cut/delete from a screenshot as to hide things for privacy reasons. As a last resort, you could use a separate user account for testing purposes. It is normal for Claws Mail to add a second header line whenever populating the first one with input. So, if you added a recipient address to the first "To:" line, Claws Mail adds a second header line as to make adding more headers convenient.