assert when a single xbl files has duplicate bindings from #405044 Q: "Would it be worth logging a xbl bug about checking if we have an existing binding to ignore any other bindings (and assert?)" jonas writes: "Yes, I think it would be a good idea to assert." note, currently, the last binding wins.

See bug 401907. If we really want to make this a warning, we should log it to the error console instead of asserting, imo. We can't assert this, because it's not an invariant our code enforces: anyone can stick multiple bindings in a file, throw it up on a web server, and point to it.

So the thing is that XBL has these kinds of assertions all over. I agree they are bad, and we should turn them into "assert if it happens for chrome-bindings, log to error console otherwise". But I'd be fine with landing this as a normal assertion for now and do the conditional assert in a separate bug.

> So the thing is that XBL has these kinds of assertions all over. And we've been working on removing them, since they interfere pretty seriously with fuzz-testing. If we're adding a new one, let's just do it right.

Pretty often there isn't reason for NS_ASSERTION, NS_WARN_IF_FALSE is enough. Do we print something to error console if document (html document for example) contains multiple same IDs? If yes, then printing to error console makes sense also in this case, IMO.

> Do we print something to error console if document contains multiple same IDs? No.