I always though it happen when you create a mail merge type thing for multiple emails when your record naming isn't consistent. If you don't use the full name then you can end up addressing someone as J instead of Mr J Smith if that is all the name you have for them.

I think the mail-merge idea is along the right lines. To the recipient, it seems overly specific, but to the person sending out the e-mail, it probably feels more natural to use a person's full name, especially if they're writing to more than one Emma or more than one Barnes.

Also, Em, what are you doing for lunch? I'm a bit hungry.

No, these most recent ones weren't mail merges, that I could tell, simply people being weird.

I am having spinach and feta filo tart with sesame seeds and a side salad of tomato and cucumber. Andy made it for my lunch and wrapped it carefully in portions in the fridge, because not only can't I cook, I can barely wield a knife without threat to life. True. Then a mini solero. Then a piece of chocolate. Then a hot drink, since it's winter now. There's enough for two but you'd have to be quick... the cats like cheese too...

Confession of a weirdo coming up. I sometimes use both names in pref to Mr/Ms where both names are known to me but I'm not familiar with the person, esp in emails where Mr/Mrs seems very formal. E.g., if I read 'for further info contact Emma Barnes at Snowbooks' (followed by email address) I might say 'dear Emma Barnes' as this seems like how I've been introduced to you and saves worrying Ms/Mrs etc. I have also used it when writing to an agent, as the agent's name 'feels' like a business name too. Thinks .. Could this be where I've been going wrong ;)
I was under the impression this was now an accepted way to approach 'unknown' people and feel sure I've also discussed this somewhere and others have agreed with me, or maybe I'm just weird!
AliB

I don't think it is ever correct to use firstname and surname in this way.
However addressing e-mails is problematic.
I would use variously Dear Mrs Barnes, Dear Emma, Hi Emma, Emma, and sometimes nothing, likewise the sign off might be, Regards, Best wishes, Thanks, Yours sincerely, or sometimes nothing
are there any rules

I often use both names when I don't know the person and it's a formal email or letter. This is partly due to my Quaker background. Historically, Quakers didn't use titles such as Mr,Mrs,Lord,Sir, as they were making a point that all people are equal, and of equal importance. Secondly, as AliB says, Emma Barnes is how you would be introduced to me, and that is what I might well put on a first email.

I've come late to this discussion but as no one seems to have explained my dilemma, I thought I'd better explain why I am guilty of this offence.

It goes like this. If I need to write or email a lady I don't know and whose details are given only as Brenda Bloggs, I have problem. I can't write Dear Brenda as she may take offence at my over-familiarity. I can't write Dear Mrs Bloggs because she might not be married or her married name is different. I can't write Dear Ms Bloggs because she might be of a generation that heartily dislikes the term. So what do I do? I write Dear Brenda Bloggs. It's ugly but it's safe--at least I thought it was until I read your post.