I'm not sure how we could control this rule, since it can be hard to tell who fired.
Or is this supposed to be an auto warning from the server?
It can also lead to some difficulties. Those who want to annoy other might simply put their own c4 on an existing pile and by rule, no one would be allowed to detonate it. If anyone would, they wouldn't stop bitching about their lost c4.
I've also seen some ppl trying to be annoying by shooting between the c4 packs and then hiting newly added packs in midair.