What does "that's you all over" mean?
I got this reply when someone asked me to do a favour, but I refused.
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It's a somewhat informal expression that means "That sums you up/That is typical of you" and is used to express a moment when an individual displays typical behaviour.
It is formed: "That + to be + subject pronoun/name + all over".
Cookie Monster: Cookies?
Me: That's Cookie Monster all over!
I haven't heard that particular phrase, but it doesn't sound too un-natural to my ear either. However, I sense the phrase's meaning as something similar to the idiom, "that's you in a nutshell". (That is a small piece of the whole you which precisely reflects the whole you.) In OP's example, OP "never" does favors for the friend and refusing to fulfill this latest favor request is typical of the friend's experience when asking for favors from OP, and the friend sums up the way they feel, "that's [my experience of how] you [behave around me] in a nutshell"
"That's you all over" has the same (That is a small piece of the whole you which precisely reflects the whole you.) image around it in my mind, except I sort of see a distinction between wrapping and wrapped. "That's you in a nutshell" draws a picture in my mind of "you" as the content contained in the nutshell. "That's you all over" draws "you" as the container, the wrapping all over the content, containing it.
That's you all over
means it's something which is typical of you or reflects one of your traits.
It can be a shortened form of
That has you written all over it
meaning whatever it is is metaphorically named after you, that you possess it
They are saying you usually will refuse to do favours.