Re: Roobet.com |Roo in Space Art Contest ~ $5000 up for grabs! |10 Winners - 1 Month
If the Roobet committee doesn't change the rules about this, I don't think it's against the rules.
As a reference to the rules based on previous contests:
Hey there Hhampuz and Roobet team. Can I submit what I already have done from previous contests? It’s perfect for the tshirt merch. I hope you can consider it. I feel like it’s designed for that. 
Yeah sure, go ahead
Better to wait for confirmation from Hhampuz for certainty.
It's good to know Hhampuz opinion, but I have not seen anyone submit duplicate works in art design competitions (there is definitely a rule in this case)