Good news! There is already a special paragraph in the TY letters for recurring donations. You can see it in the footer here, enclosed in [ifRecurring] [endIfRecurring] tags.

https://meta.m.wikimedia.org/wiki/Fundraising/Translation/Thank_you_email_20170805

You can use those tags in the body of the email as well.

Cool! Thanks for sending that, it's helpful to see which merge fields we're using. Is it possible to show a different message where donation count > 1?

We'd like to add language that acknowledges someone has given more than once if we know they have. The =1 TY letter wouldn't refer to it being their first donation in case there are duplicate records.

As discussed in standup:
The difficulty with repeat donors is that we don't de-duplicate before sending the TY letter. There's a ticket someplace to pass the contact ID from email clicks all the way through the pipeline so for at least those donors we can add the donations to an existing record and not create a duplicate.