Hi Maggie,
I believe you could swap the two send email flow steps and add a choice to the first one to make sure you are not sending Email A twice. See below for an example.
![0EM50000000SeoM.jpg 0EM50000000SeoM.jpg](/t5/image/serverpage/image-id/15159i30BD4D3113E6B32F/image-size/large?v=v2&px=999)
Choices in flow steps will pull out the first matching choice, so anyone who received Email A previously will receive email B, and everyone else (leads entering the flow after your edits to the campaign), will receive email A.