Zebra - also a horse

A string $s$ is defined as a zebra if it can be divided into several substrings s = x_1 + x_2 + \ldots + x_k (k \geq 1) such that the following conditions hold:

• Each substring x_i consists of identical characters;

• No two substrings x_i and x_j (i \neq j) share the same character.

For instance, the string "aaabbccc" qualifies as a zebra, whereas the string "aabbbaa" does not.

You are provided with a string. Determine if it is a zebra.