>Can anyone give me a simple idea how to remove the duplicated letters in a text
Can you be more specific? Should the two L's be removed in "SLOWLY"? If so, which L should be removed? Are the characters to be ordered in any special way? All of these play a part in figuring out your solution.
Store the characters you've found in a suitable data structure and check the data structure with each new character. If it was seen before, don't do anything, otherwise add it to the data structure and print it. Here's an example using the standard set class:
ivailosp, that only removes consecutive duplicates. You failed to handle the specific case that I asked about, where "slowly" becomes "slowy".
>what is #include<set.h>??
It's a standard header for the set class (but note that it's <set>, not <set.h>). The set class is basically a data structure that allows you to store and quickly search for items.