You start by using a negative lookahead to prevent a match if the captured part repeats after the colon:
(\p{L}+):(?!\1)
Then you need to match the second unicode word, another \p{L}+
:
(\p{L}+):(?!\1)\p{L}+
And last, to prevent false matches, use word boundaries:
\b(\p{L}+):(?!\1\b)\p{L}+\b