With some configuration, attachments are joined twice.
Way to reproduce it:

• Create your own command to send the email.
• Set in the context all the parameters needed to send an email, that includes from(test@email.com), to(test@email.com), type(freemarker), text(rendered body from freemarker file) and finally attachment (instance of MailAttachment).
• The email is send but with two same attchments

So the current process is:

• Create an instance of the email depending of the type (for this case freemarker)
• Some others actions doesn't influence the process
• set the body from resource file and as freemarker email extends html Email it sets the attachment
• Then come back in the email creation process and set again the attchment

A way to solve this problem is to not add the attachment when the body is set and only after.

Look at the patch