I can see your predicament. However, copying an idea (not that I am suggesting that you are 'copying'!) does not infringe copyright in someone else's work; but if your expression of the same idea resulted in the same or similar work to the original, then the fact that you were aware of the other person's work would tend to count against your reasonable defence of independent creation. Much depends on what type of work you and the other designer are dealing with. If you have plenty of leeway to find another way of making the idea work which is clearly different to the other person, I don't think you need worry. The classic case on that sort of facts was Baigent v Random House
(the so-called Da Vinci Code case) in which Da Vinci Code
author Dan Brown was accused of infringing the copyright in an earlier book known as The Holy Blood and The Holy (HBHG). While it was admitted that Brown had used HBHG as part of his research, he won because the court found that although both books dealt with very similar subject matter, the expression of those ideas in Brown's book was not similar to HBHG.
An example of the decision going the other way is a case known as the Red Bus case
. Here again the defendant had had prior access to the complainant's work, and unfortunately for him, had also lost an earlier infringement claim on much the same issue. On the second occasion the defendant set out to copy the idea, but to find a different way of expressing it, although the court was unimpressed by this argument and found there had indeed been infringement.
The other thing which concerns me is that you have used the word 'designer', which implies that the work you were sent (and the one you intend to produce yourself) are designs in the sense that design right
may be applicable. If so then this poses serious difficulties because design right is a monopoly right, meaning that there is no defence of independent creation, and it doesn't matter whether or not you were aware of the other design before bringing out your own design. Assuming that a reasonably informed user forms an overall impression that both items resulted from the same design, then that is enough to establish infringement. Things like surface decoration may or may not count, depending on whether the other design has been registered. Where you have less leeway in making design differences - perhaps because of technical constraints in manufacturing or the end use of the item - then the more tolerant the court is likely to be in accepting similarity. The classic example here is of replacement car parts such as hub caps or bonnet badges which have look similar to the originals in order that the car appears correct after repair etc.
So having possibly given you even more bad news, I think your best course of action is to politely decline the on-spec design by saying that you are already working on something similar (there is little to gained by withholding this information; if you do produce your own design the other designer will no doubt assume that you have copied theirs anyway).