I'm pretty sure you don't have to take the test again if you loose the pass certificate. 
They can't be replaced because they are stamped and signed by the official at the test center, but each certificate has a unique identifying number and as I understand it the pass result is electronically forwarded to UKVI.
I think that's also the reason why you can't book an in-person appointment for ILR within 2 days of passing your life in the UK test, so that the result has time to come through to them.
I did a bit of searching online too and found some anecdotal evidence that if you have lost the certificate, it's not a problem, you don't have to re-sit the test.
It's easier if you have a note of the certificate number but from what I read, even without that, they'll be able to dig up your result.
But yeah, best not to loose it in the first place and definitely scan it/copy it/make a note of the number somewhere else, just in case you do!
Correct, they keep the certificate electronically now.
Ah, okay - thanks, guys
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I was getting myself confused... previously (a few years ago), you had to retake it if you lost the certificate. Then a few months ago, I saw through posts here that you didn't have to retake it and could get a new certificate.
However, when I checked online again the other day before answering in this thread, I saw you couldn't get another certificate, and from what I was reading, it sounded like you had to retake the test (the UKVI website is very vague on the subject but another website said you did have to retake).
