Those two dashes are added by Gmail. It is a separator of a signature defined by email standard. Despite this fact, you can get rid of it with a setting in Gmail:

  1. Go to Gmail settings: https://mail.google.com/mail/u/0/#settings/general
  2. Scroll down to Signature
  3. Right under your signature, tick the checkbox followed by text: Insert this signature before quoted text in replies and remove the "--" line that precedes it
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