This is annoying, because "Yes, yes" can either mean added emphasis, or dismissal.
It's a context and tone thing, which is really difficult to discern in writing; it's much easier to figure out which is meant when you're speaking with someone aloud or in person.
Punctuation can help:
"Yes, yes! Let's do it!" -> "I am so excited about this I had to say yes twice!"
"Yes, yes, we know, you're very smart." -> "I am bored by this and want the conversation to move on."
It's very idiomatic and really depends on the tone. In your example it's probably the former meaning. The exclamation point and the "plus one" meaning an upvote probably mean they are enthusiastic.