Generally, the CR of any Class Level NPC is the same as its level minus 1.
So the CR of a 10th level NPC Fighter is 9. The CR of a 10th level NPC Wizard is also 9.
CR increases differently when adding multiple creatures. Two CR 9 creatures are not CR 18, It's 11. Counting from 2 to 6 (the typical party sizes):
- Creatures: 2 CR: 11
- Creatures: 3 CR: 12
- Creatures: 4 CR: 13
- Creatures: 5 CR: 14
- Creatures: 6 CR: 14 (no, that's not a typo. I'll explain later)
CR calculation is a complex business and you can read all about it here. I'm going to try to pick out details that are important in a doppelganger fight.
APL and relative CR encounters
A party's APL is
- ((sum of levels)/number of players)-1 for parties of 1-3
- sum/number for parties of 4-5
- (sum/number)+1 for parties of 6.
This is because the number of "tools" available varies by the number of PC's.
CR is directly compared to APL when determining how dangerous an encounter could/should be. CR's less than the party level (effectively everyone fighting a single character of an equal level) is considered Easy. the scale ends at an encounter with CR+3 compared to APL which is considered Epic and has been described as 'at least one player will probably fall'. As you can see, CR isn't linear; it's but actually based on the amount of Exp awarded, so it roughly scales on a negative harmonic progression (wiki link, not important).
One more rule specifically applies, about Gear Adjustment. NPC's have a lower Wealth by Level threshold than PC's and the CR assumes that is followed. If an NPC has no gear (assuming it hampers them) has its CR further reduced by 1. An NPC that has gear equal to a PC has its CR increased by 1.
This alters my previous bullets as so:
- Creatures: 2 CR: 12
- Creatures: 3 CR: 13
- Creatures: 4 CR: 14
- Creatures: 5 CR: 15
- Creatures: 6 CR: 15
The doppelganger concept
Assuming all of this applies directly, CR of an equal copy of the party will always be
- +3 for a party of 2
- +3 for a party of 3
- +4 for a party of 4
- +5 for a party of 5
- +4 for a party of 6
While this assumes combat readiness and the PC's wont have a full regiment of spells ready for the doppels to take, any decrease from loss of spells is equally reflected in the PC's and therefor ends up a null change.
These numbers suggest that this is a bad idea. As you probably guessed, the doppel's have an equal likeliness of winning the fight compared to the PC's.
It simply shouldn't be done. There are a couple ways to make this an interesting and engaging fight. You can use one, multiple, or all of them depending on your intent.
- Give the doppelgangers a weakness the party can exploit that the party doesn't have. Make them take 1.5x or double damage from Cold Iron, Silver, Good, or an element. Provide the party some way to divine this and pepper it throughout their last couple fights.
- Give the party a tactical advantage, such as the doppel's aren't fully capable in their adopted bodies yet and can't do a list of things, or always move as though they're on difficult terrain. Have them hampered by the actual environment, or slap a couple Conditions such as Sickened on them.
- Don't play the doppel's as intelligently as your PC's play their characters. Do 'goof up' things like provoke AoO or use low level spells first.
Finally, consider letting them copy everything except all of their magic items are not magic. That looks like a Headband of Vast Intelligence +4, but on the doppel, it's actually just a fancy sweatband. This would drop their CR by two each resulting in my final encounter difficulty table:
- Creatures: 2 CR: 10 (APL +1)
- Creatures: 3 CR: 12 (APL +2)
- Creatures: 4 CR: 12 (APL +2)
- Creatures: 5 CR: 13 (APL +3)
- Creatures: 6 CR: 14 (APL +3)