Company I interviewed with called to ask me what I was thinking

I interviewed for a company a few weeks back and got an offer right away (woo!). I have an interview with another company this week and was just called by someone from the first company asking when I would get back (they gave me until the end of the year). I told them I was going to an interview with a different company, and would wait to see if/what I was offered there. Was this the right thing to do? Should I have disclosed I am going through with another interview? It really is a toss up between the two companies right now.

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