I have an audio file of an interview with a person. Unknown at the time we recorded, the mic was picking up his voice out of a speaker and thus there is an echo throughout.
 
I'm looking for ideas on how to get rid of the echo. Things I have tried, but which had limited results, include:
 
1) EQ on a very narrow freq using a high Q (better but not good enough).
 
2) Noise gating

You can download a trial version of Zynaptiq's Unveil...see if it works.

I tried a demo of Sound Soap once and it does amazing things with noisy background. But I think your issue might be impossible to clean up. Noise removal software looks for,, well noise. 
A repeated word is not really noise. 
Is it only noticeable in the pauses between words? 
If so if a noise gates threshold can't nail it then you might be faced with manually running through the track and silencing the pauses. 
I've done using Wave Lab. 
 
You can almost see the places where the pauses and breath is with a wave editor set at the right zoom level. All you need to do is highlight the offending space and hit backspace and it will silence that part. If the spaces are large I also cut time out of them. 
 
You can do wonders with interviews by tightening up the spaces. A 10 minute speech, sermon can be cut down to 8 min etc. Cut out all the pause to think words like --you know and uhhhh ..you can make someone sound more confident.   

guyshomenet
I have an audio file of an interview with a person. Unknown at the time we recorded, the mic was picking up his voice out of a speaker and thus there is an echo throughout.
 
I'm looking for ideas on how to get rid of the echo. Things I have tried, but which had limited results, include:
 
1) EQ on a very narrow freq using a high Q (better but not good enough).
 
2) Noise gating

Unless the echo is so far away from the original voice that you can use Volume Automation to get rid of it, there is nothing you can do that will make it go away 100%. Excuse my bluntness, but in recording there is a saying: Crap in equals crap out. This is just another lesson learned in the number of mistakes we have all made during our careers and WE HAVE all made them, sadly
 
Things to do to avoid it next time:
Next time do a soundcheck and listed to the recorded signal before going on with the interview. you would have caught the issue and you could have corrected it before you recorded the interview.
 
CJ