Don't let players use the exact same word multiple times!!! Just because they add an 's' or another noun doesn't mean it should be accepted!
You are correct and adding an ‘s’ should not count it as a separate word. If you could send support a ticket with the word/category we can remove the word from the database and it will then only accept the singular version of the word.
Adding other nouns after the first word has been decided to be permitted in the mobile version.