Fix: remove duplicate word suggestions from multiple dictionaries - #82

Description

This PR fixes an issue where duplicate completion suggestions were returned if the same word existed across multiple dictionary files.

Problem

When multiple dictionary files contain overlapping words, the plugin includes the word as a suggestion for each file it appears in, resulting in duplicate entries in the completion menu.

How to Reproduce

1. Configure the plugin with multiple dictionary paths:

local paths = {
    "/tmp/dict1.txt",
    "/tmp/dict2.txt",
}
require("cmp_dictionary").setup({
    paths = paths,
})

2. Add overlapping words across the files:

/tmp/dict1.txt

red
blue
green

/tmp/dict2.txt

pink
blue
yellow

3. Trigger completion for blue.

• Expected: A single blue suggestion.
• Actual (before fix): Two blue suggestions.

Solution

Added a filtering step using a local set (seen) to deduplicate completion items by label before returning them. blue (and any other repeated word) now appears only once.