I started an unmodded game of the original Baldur's Gate and something strange is happening, at least I think it is strange. If I roll a single class character when the smoke settles after Gorion's scene and you meet up with Imoen she is a level 1 thief.

However if I roll a multi-class, I've tried fighter/mage and cleric/ranger, when you meet up with Imoen she has 1304xp and is a level 2 thief. Is this normal? I was planning to dual class Imoen at level 3 but when the game levels her up she puts points into stealth making a level 3 dual impractical.

As I said this is a completely vanilla game downloaded from gog.com, is this normal?

Yes, it's completely normal. The game is scripted this way. I'm not sure what are the reasons of this level adjustment (any ideas?), but this is how it is. If you don't mind using a cheat, you can try this to spawn a level 1 Imoen: https://www.gog.com/forum/baldurs_gate_series/vanilla_bg1_multiclass_pc_imoen_joins_at_level_2xp_1304_how_to_get_level_19_xp_version/post5.

Thanks for the answer!

I will probably just use a save game editor to reshuffle Imoen's thief skills.

When companions are spawned their level is determined by your MC's level. If your MC is at level 2 Imoen will also be level 2 at that point. Pretty sure when you're multiclassing like that, the game treats you as having 2 levels.