#!/bin/bash
s='Conclusion
cold
Collaccio'
grep -i '\([[:alpha:]]\).*\1' <<< "$s"
echo "---"
grep -iP '^(?!.*(\p{L})(?:.*\1){2}).*(\p{L}).*\2' <<< "$s"
echo "---"
grep -i 'c.*c' <<< "$s"
echo "---"
grep -i '^[^c]*c[^c]*c[^c]*$' <<< "$s"