History suggests that isn't the case. Heljan would in any case be looking to sell to a wider market, via their dealer network. If the DJM model arrives first and is available in sufficient quantity, Heljan will probably need to compete on the basis of quality and price.
There've been numerous examples of different manufacturers taking on the same prototype, most recently Dapol and Bachmann doing the twins. Dapol's commission for Hattons arrived first, but both sold very well. And that was with a class of only two locomotives.