Is there a command to clone entities?

With /summon command you can make any amount of same entities you want. There is no command that would take entity A and make entity A'. You always need to create new entity in vanilla Minecraft.

However if you want to take entity A and re-summon it somewhere else, you can. All you need is to have the command what summoned that first entity and reuse it (for example via redstone clock). And if you want to reuse it on custom places (for example summon entity on 5 marks at time), you can with help of /execute command.

Wiki: Execute, Summon, Scoreboard, Summon generator examples : 1 , 2

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So, let's say you have 4 player party and you want to face them some equal mob challenge. This command will summon one mob for every player which will be targeted(for example everybody in "arena", so on some coordinates with some range):

/execute @a[x=100,y=64,z=100,r=30] ~5 ~0 ~0 /summon Spider ~0 ~1 ~0 {CustomName:Tough Spider,CustomNameVisible:1,Health:100.0f} 

This command will summon one spider for every player, 5 blocks away from that player in X direction.

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Another example: You want to summon 4 zombies surrounding the player when he enters the centre of the room. Then you need (aside from commands checking when he enters the area - or redstone):

• a) 4 same execute+summon commands, every one pointing to another coordinates
• b) prepared "marks" to execute one execute+summon command on them as we done before on players

Marks can be done by Scoreboards and some dummy entities. For this example you can prepare summoned 4 invisible ArmorStands on places where the command shall run and tag them (tag is optional for secure executing - and distinguishing the targets for multicommand uses in one area - like one armor stand will summon zombie and another will summon skeleton)

Then all you need is to target ArmorStands with /execute as before:

/execute @e[x=100,y=64,z=100,r=30,type=ArmorStand] ~0 ~0 ~0 /summon Zombie ~0 ~1 ~0 {CustomName:Tough Zombie,CustomNameVisible:1,Health:100.0f}

This will summon one zombie on every prepared armorstand with one command. However you need multiple commands to prepare it.

Also you can target already existing summoned entity, however you need other command for first zombie (basic /summon on coordinates or on mark) and then execute+summon command to multiply it:

/summon Zombie 100 64 100 {CustomName:Tough Zombie,CustomNameVisible:1,Health:100.0f}
/execute @e[x=100,y=64,z=100,r=10,type=Zombie] ~0 ~0 ~0 /summon Zombie ~0 ~1 ~0 {CustomName:Tough Zombie,CustomNameVisible:1,Health:100.0f}

Those two commands will summon two zombies on same spot. Also you can put second command on redstone clock, executing it more times, so there will be more Zombies (but be sure to not spawn them fast, they can multiply exponentially and lag/crash the server!)

Also you can use scoreboards for the last thing, by tagging entities and adding them score for every command you do on them (for that you need chain commandblock that will operate the scoreboard) and then run command dependant on scoreboard score, as described here.

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Hopefully this can help you to get what you need. And basically it answers possibilities to do this thing with only one command - with some prepared/basic situations, it is possible.

PS: Since 1.9 version, you can use chained commandblocks with ease, therefore it is really easy to use multiple commands that will execute just after each other, just by pressing one initial button. They are compact and can fulfill multiple tasks really easy.