I have beenwas suffering from the same problem. The solution I preferchose is to disable auto-save-mode in mu4e-compose so that drafts do not accumulate.:
(add-hook 'mu4e-compose-mode-hook #'(lambda () (auto-save-mode -1))) Plus obviously, set mu4e-sent-messages-behavior to delete, as suggested by previous answers (especially in Gmail since GamilGmail takes care of sent messages):
(setq mu4e-sent-messages-behavior 'delete)