Is saying "candor and honesty" grammatically correct?

I recently read the sentence

Thank you for your candor and honesty.

Is that grammatically correct? To me it seems redundant as candor is a synonym for honesty. Does candor have other meanings that would not make it redundant in this sentence?

The question you're asking isn't really grammar-related as redundancy isn't a grammatical problem (so, yes, it's perfectly grammatical).

As far as being redundant, while candor does relate to honesty, it usually has a sense of being not only honest, but direct, frank, or otherwise outspoken. So it's quite possible to state something honestly, but not particularly candidly by beating around the bush or being especially tactful.

"Candor" means frankness or openness, "honesty" means truthfullness.

As Dusty notes, you can certainly be honest without being candid. If you are reluctant to tell the truth and it has to be pulled out of you, or if you beat around the bush or use euphemisms, for example.

Whether you can be candid without being honest gets into the exact definitions, or maybe connotations, of the words. As I understand them, if you said, "I was really mad because Bill insulted me," that would be "candid": you are freely and openly expressing your opinions and feelings. But if Bill's insult was trivial, or if it was prompted by some bad action on your part that might lead an outside observer to say that you were to blame, then your statement is not really honest. So in that sense I think you could be candid without being honest. It's a fine line, though.

In any case, sure, I try to avoid being redundant or repetitious or saying the same thing twice. But redundancy is not necessarily a bad thing. It is often used for emphasis. If you say, "Sally is an incredibly beautiful, stunning, gorgeous girl", you might say that technically this is the same as saying, "Sally is pretty", just redundant. But clearly the first sentence is more emphatic than the second, precisely because it is redundant.