The input is a word of lowercase letters not separated by whitespace. A newline at the end is optional.
The same word must be output in a modified version: For each character, double it the second time it appears in the original word, triple it the third time etc.
Standard I/O rules apply. The shortest code in bytes wins.
Tests provided by @Neil :
tutu -> tuttuu queue -> queuuee bookkeeper -> boookkkeeepeeer repetitive -> repeetittiiveee uncopyrightables -> uncopyrightables abracadabra -> abraacaaadaaaabbrraaaaa mississippi -> misssiisssssssiiipppiiii