I've heard from numerous Christians that "A true Christian can not deconvert" and that once somebody is "saved" they are always "saved".
These people will usually tell a deconvert that they never were a True Christian. They just can't accept that a True Christian could leave the fold.
Does this phenomenon exist within Muslims or is it unique to the Christian religion?
Well, I think scripturely they do have a point since it seems that that is what their scripture teaches.
With regards to Islam, in Islam people can do what they want, someone may know the truth of Islam and leave it, if theydo that then I dont know if they would be a Muslim, a Muslim is a person who does Islam, submission to the will of almighty G-d.
I have seen nothing to say that a person who is a Muslim cannot leave Islam, rather what I have seen is that whoever G-d guides none can misguide and whoever G-d misguides none can guide.
That is all, but you see we all have the free choice.
Having said this, if a Muslim does not take care of him self then doubt can happen and so forth and he can leave Islam, but thats more of a self imposed thing, this is why in my view, I see alot more reverts in the younger age groups practicing Islam than alot of those who have been brought up that have taken Islam for granted so they do not pray that they will be good muslims or that they will have the gift of faith, because if G-d wants he can take your faith away.
I hope that's kinda answered.