Closing.

'looks like you've solved it'

Not an issue

You should return instead of console.log. Always assume you should return the answer because AFAIK, CodeWars doesn't read from console ever.

You should always assume you should be returning the value because CodeWars doesn't read from the console ever, AFAIK.

Your value needs to be returned, not logged out. Though upon my review, I did not see that having to return the value was included in the description. So, +1 for adding that to the description.

Just note that with the answer by @the-code-less-traveled it matches both letters and numbers (not just letters as s/he specified on the last line).

This is mostly right but I would like to offer a couple of small corrections:

* Means to match the preceding expression zero OR MORE times. In this case where it follows '.' it allows for any number of random characters.

\1 Refers back to your match for the last expression in parentheses. In this case this means the character that matched '(\w)'.

In total '(\w).*\1' equates to: An alphanumeric character, '\w', followed by any number of random characters, '.*', followed by the same alphanumeric character again, '\1'

Hi PantlessBandit! I'm going to try to explain this since I just got finished looking up how this works.
What chunjef created is known as a regular expression, or regex for short. Regular expressions are a way to describe an analyze string data.
Regular expressions use forward slashes to encapsulate what it is we're looking for in the string.
\w (with a lower case w) means to look for an alphanumeric character
. represents all other characters while * means zero instances
What this means is to leave all the other characters alone like digits.
Finally the 1 means that there should be only 1 instance of each letter.
The i makes the data analysis case insensitive
The ! means NOT
So, to put it all together
Test the string to see if any LETTER (not other characters) does NOT appear more than once.

I hope I explained that right. I just finished learning about it here:
http://eloquentjavascript.net/09_regexp.html#p_uNMQxzr01n

Your code counts "I'm" as two separate words. It's one word. Same for other non alphabetic charcters.
Also, You can use console.log to print the arguments to your function. It would make finding bugs much easier.