Not able to submit transactions in queue

[ItsAditya-xyz]

I am getting error every time I submit transactions in queue.

I have mass unfollow, thread builder, mass creator coin buy/sell feature in my app which used to work perfectly few days ago.
(Users were even able to make 5000+ unfollow transactions without any problem)
But now, the second time i submit a transaction, I am getting this error

{
     "error": "SubmitTransaction: Problem processing transaction: VerifyAndBroadcastTransaction: Problem broadcasting txn: 
     BroadcastTransaction: : Server._handleTransaction: Problem adding transaction to mempool: : tryAcceptTransaction: Problem 
     connecting transaction after connecting dependencies: : ConnectTransaction: : _connectSubmitPost: : 
     RuleErrorInputSpendsPreviouslySpentOutput"
}

Any idea why this is happening now?
@diamondhands0 @lazynina

[lazynina]

This error happens when the previous transaction hasn't hit the mempool yet. Basically you're constructing a transaction that's using UTXOs that were spent by a previous transaction. Use the get-txn endpoint to make sure the txn has hit the mempool before constructing the next transaction