You are given a string s consisting of lowercase letters. Your task is to remove duplicate characters from s such that each character appears only once. Among all possible results, return the one that is the smallest in lexicographical order.
The order of the remaining characters must preserve the relative positions of their first occurrences in s.
Input: s = "ecbacd"
Output: "ebacd"
Explanation: By removing duplicates and maintaining lexicographical order, the result is "ecbacd".
Input: s = "aaaabbbbcccc"
Output: "abc"
Explanation: After removing duplicates, the smallest lexicographical string is "abc".
Input: s = "zxyzy"