If your item concept is same as yours and its newer than your script and reply on same Envato platform then Yes you can take any legal action on that author.
Contact envato for that and without prior DMCA notice [i.e because item rely on same platform envato] you can perform action on that author. But do so please collect all the information where you can prove that you are the real developer for that script [i.e script idea].
OR take a look at https://help.market.envato.com/hc/en-us/articles/202822720-Sending-a-DMCA-takedown-notice
and
http://envato.d.pr/1cpls/5utNh9lX
Regards,
Phoenixcoded