Sadly, this sort of thing has happened and continues to happen. At least once it created major and very OOC trouble. That particular instance has been resolved, but there certainly are many such cases going on, not known to me.
The only correct behaviour IMO is that if you play an alt, be it on the same account or another, and you start to interact with someone you or your other character know, to immediately tell them that you are playing an alt of the char they know already.
The only exception is the use of very temporary alts as parts of an RP that you are conducting, where their purpose is short-lived and interaction doesn't take place beyond the replies to quest-related issues, because in that case, players are likely to know beforehand that they are alts of you.
Why would players not do this? Several reasons:
1) To spy on IC things. Very OOC and IMNSHO should be punished.
2) To spy on OOC things. Must be punished severely. This is close to fraud.
3) To remove bad OOC reputation. Instead of being honest and trying to better your reputation, they take the easy way out (which won't work, unless they actually changed their ways, in which case the disguise wouldn't be required).
4) To abuse kindness of other players. Like 1.
5) To gain undeserved OOC attention of whatever sort. Like 2.
In short, with the single exception I mentioned, I see absolutely no valid reason why this should be acceptable.
IC, however, any new character must obviously know only what they learn, and transferring knowledge from one char to another is bad RP at least. You can always skip things that your char may have learned already (like landmarks and such) if it doesn't add to the RP at hand, but other things, like other chars (not players!), must be gotten to know separately. This isn't much of a burden IMO, because alts shouldn't happen frequently.
I have only one single account, and this single account has two characters: one for RP (Netrhys) and the other (Seytra) solely for testing, but if a player thinks they can RP several characters, this is fine as well (though not at the same time, for the delayed responses).