Python 2, 116 bytes

def f(s):a=s.lower();return''.join(`n`+s[a.findrfind(c)] for n,c in sorted((a.count(c),c)for c in set(a)if c.isalpha()))

Python 2, 115 bytes

def f(s):a=s.lower();return''.join(`n`+s[a.find(c)] for n,c in sorted((a.count(c),c)for c in set(a)if c.isalpha()))

def f(s):a=s.lower();return''.join(`n`+s[a.rfind(c)] for n,c in sorted((a.count(c),c)for c in set(a)if c.isalpha()))

added 24 characters in body

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edited Jan 18, 2019 at 21:50

Python 2, 99 bytes

Python 2, 115 bytes

def f(s):a=s.lower();return''.join(`n`+s[a.find(c)] for n,c in sorted(`a(a.count(c)`+s[a.rfind(,c)]forfor c in set(a)if c.isalpha()))

def f(s):a=s.lower();return''.join(sorted(`a.count(c)`+s[a.rfind(c)]for c in set(a)if c.isalpha()))

Python 2, 115 bytes

def f(s):a=s.lower();return''.join(`n`+s[a.find(c)] for n,c in sorted((a.count(c),c)for c in set(a)if c.isalpha()))

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created Jan 18, 2019 at 8:03

def f(s):a=s.lower();return''.join(sorted(`a.count(c)`+s[a.rfind(c)]for c in set(a)if c.isalpha()))