If you put more than 1 shard into an anvil, you can potentially dupe them. This is because the output is equal to the amount of shards you put in, but it only decreases the input by 1 when you take all the shards out.

With the picture as an example:
I put 4 shards in, take 4 shards out.
3 shards remain in the input slot. I take 3 out, I now have 7 total.
2 shards remain in the input slot. I take 2 out, I now have 9 total.
Then I take the last one for a total of 10 colored shards.

I'm playing on the 1.18.1 version

So on further testing, it seems to be my modpack that's influencing this behavior. I'll reply when I've found the source of the conflict. If you have any idea what might be the cause, please let me know.