If you're using dice, this is a roughly 50-50 fight
With the same statistics and abilities, and randomness in the form of Dice, cards, or something else, you have a roughly 50% chance of a TPK, and nearly guaranteed at least one player will die. This is probably way above the normal encounter guidelines for any system to consider a "Deadly" encounter.
If the invoker participates in the fight, it's almost assuredly a TPK
If the invoker isn't doing this as a stalling technique to flee, or in hubris because he thinks his clones will win, this encounter will mop the floor with them.
Additionally, this raises a verisimilitude concern: if the clones are identical in strength and the Invoker flees, why didn't he simply stay, since he would nearly guarantee he would have the upper hand and be victorious.
For these reasons, you should make the clones easier to defeat than the players, whether the Invoker flees or not.
You will have better macro tactics, they will have better micro tactics
You will have full map knowledge, be able to make all the mirror-PCs act in a well orchestrated and coordinated strategy, while knowing all the weaknesses of the players. This will weight things in your favour.
They will not immediately know these are 100% identical clones, nor will have the map knowledge.. But they will have much more intimate and familiar knowledge with their own characters and their team. By now they should have developed successful tactics for their characters and team. This might weight things a bit more in favour of the players.
If you want for it to be a fight to the death where anyone could win this is fine, but if not...
Solution 1: The clones are imperfect copies
The clones could simply be highly similar based on what the invoker thinks he knows about them. Perhaps remove a few abilities, stat points, or powers, and reduce the HP somewhat to weight things in the players favour.
In a similar vein, simply giving the clones half the HP, cutting any daily abilities and having the Invoker also participate in the fight could make for a very challenging encounter. This depends heavily on how difficult the Invoker would be for the party however.
This should let the PCs have more options and resources to survive the fight, but it can still be very brutal and challenging mirror match.
Solution 2: There is a way to short circuit the fight
Perhaps attacking the invoker directly will cause him to lose concentration on the clones, making one vanish. Maybe the clones are summoned by various totems or wards that can be destroyed by the players, fizzling a clone. Maybe the players can use a Banishment spell to instantly destroy one of the clones on a failed save.
This would allow the players to swing the numbers into their favour without having to fully defeat a summon, swinging the advantage again to the players.
Note that, if you do go this route, be sure to telegraph these mechanics to the players in multiple ways, because the stakes are very high, and if they don't figure it out they will likely die.
Solution 3: Break this into multiple encounters
Either the players come across a subset of their clones and a few other minions as they approach the Invoker's lair, or the Invoker cannot summon all the Clones at once, but rather over the course of 2-3 rounds.
If you have small subset fights, you can theme them strongly to the cloned PC's present. Maybe you have an encounter for each Clone (or a pair), with a group of minions, and a battle arena that fits their play style. This would be great for highlighting each Character as you go as well.
If the Invoker must summon them slower, the PCs have a chance to focus fire the clones or Invoker down before the encounter hits critical mass.
Solution 4: Make the clones ideals not identical
Instead of making them exact clones, make them ideals of the characters. Take their best aspects, and worst aspects, and exaggerate them. A strong melee fighter, who is a bit dumb, might become a ferocious but nearly braindead melee combatant.
If doing this, you'll still need to use one of the other solutions in tandem to reduce the danger of the encounter. Making the clones have worse overall stats being one of the best meshing strategies. You make their best and worst aspects be obvious, but everything else disappears.
No matter what: Playtest the encounter
If you don't playtest the encounter, you will have no idea whether the encounter will be too difficult for the players, or whether it will be frustrating or just plain unfair feeling.
You can do this solo (playing against yourself) or recruit some other friends not in the campaign to help out to get a good feel of the balance.
Make sure you run it at least 2 or 3 times due to the swinginess of the combat.