Only one
s in
present!!!

You can't use will with the stative verbs in this construction: [Let's see / Let me know / Tell me] plus an indirect question. I would not call these relative clauses.
to finish first - an action.
how you are - a state - not an action.
get there first - same as arrive first - an action.
you think - an internal activity; having an opinion - not an action.
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Thus, with the indirect question in brackets { }, ungrammatical choices prefixed with asterisk *:
[Let's see / Let me know / Tell me] {who [finishes / will finish / gets there / will get there] first}.
[Let's see / Let me know / Tell me] {how [you are / *you will be / what you think / *what you will think]}
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Your example with make sure does not belong to the pattern above. There's no indirect question.
Make sure (that) you come back soon.
A make sure that pattern is not followed by a future (will).
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You can save yourself a lot of headaches by using the present tense for all of these. Even in cases where will is possible, it's almost always the inferior choice.
CJ