my player who is a warlock has the spell resonant echo

You tap into the unseen futures and pasts, calling forth a shadowed copy of yourself within range.
This copy rolls it's own initiative and acts as you would on its turn (you choose the action it takes). This copy can only take the attack action to cast one spell or it can use its reaction to absorb one weapon attack directed at you if the copy is within 5ft of you.
Any amount of damage shatters this copy and it fades from existence once it casts a spell.

now does this mean this warlock can pop multiple spell slots to have multiple echos at once? judging by the wording it seems like they can

Seems like a homebrew, I think. 
Personally, I’d not allow it. Poorly worded, and overpowered (probably, but who can tell with the way it’s worded).

Like above, this feels a bit homebrew. That said, if it was ballance, then it follows Familiar rules, so any spell you cast through it would count towards the number of spells you can cast in a turn. So if it casts a leveled spell, you would be stuck casting a cantrip at best.

What level is it? Cuz if it's a higher-level slot like 8-9 then maybe, but anything lower than 5 is a hard no from me. Or you could modify it a little bit to only spawn a certain number of them to balance or just one.

in the end it all depends on how powerful you're allowing your PCs to get in accordance with the enemies you're throwing at them. 

It looks like a version of a critical role  spell but not one of the spells published here. I also can't find the exact wording any where so this is likely a recreation.

Based on how resonant echoes works in  critical role steams and what is written here, my understanding is that you could cast two spells on one turn but the echo would vanish when you do.  In other words you spend a spell slot and an action to gain an action on a later turn that you could use to cast a spell which is a net loss in terms of resource efficiency and action economy. In other words you are making a spell more expensive in order to cast less spells at a later time. 

The main advantages are tactical as you could use the echo to cast spells from a different location without risking your caster or time two spells  to go off at the same time.  The former is a weaker version of invoke duplicity and the later could be accomplished with team work.  Given invoke duplicity as a reference point it's appropriate for a level 2 caster so would be a level 1 spell. 

However, looking only at the streams it isn't clear to me that the description given here is correct.  The spell critical role uses may summon a specific monster called an echo which has specific spells/abilities as well as hit points/ ac.  In which case I can't judge it's power without seeing that stat block. 

edit: I almost forgot the damage reduction. That's actually the most powerful part of the spell. I'd balance it the same as false life and cap the damage reduction at an amount equal to that spells temp hit points.

so funny thing is i knew this spell was critical role so i just assumed it was in the critical role books that i have bought on this site, it is in fact a homebrew that i had no idea nor memory of ever adding to my collection i just assumed it was an official spell, well regardless luckily the campaign that my player who has this spell is in is on the final fight so after the campaign is wrapped up ill take a deeper look into it and rather get rid of it or tweak it, thanks for the info guys