How to examine and change actions on commercetools updateCart query?

My question is this:

Is there a feature or method within graphql that allows editing of incoming queries EASILY so that I can prevent an action from being performed? I would like to examine my query in a web proxy, edit the actions, and then send it on to its destination.

Context:

I have a mutation running in a proxy service for commercetools that looks like this:

mutation AddLineItem(
  $id: String
  $version: Long!
  $sku: String
  $quantity: Long
  $currencyCode: Currency!
  $centAmount: Long!
  $custom: String!
) {
  updateCart(
    version: $version
    id: $id
    actions: [
      {
        addLineItem: {
          sku: $sku
          quantity: $quantity
          externalPrice: { centPrecision: { currencyCode: $currencyCode, centAmount: $centAmount } }
          custom: { typeKey: "custom-type", fields: [{ name: "field", value: $custom }] }
        }
      }
    ]
  ) {
    id
    version
    lineItems {
      ...LineItemFieldsCart
    }
  }
}

According to the commercetools documentation, when the external price field is set, duplicate items will be added as separate line items to the cart instead of increasing the quantity, as is the default behavior. The thing is, I want the default behavior instead of the duplicate line items. The normal way to mitigate this is to add an update extension to delete the duplicates, but I am working on an already mature system that has multiple update extensions that all operate on the line items, making a delete apparatus a heavy lift in the update extension. I want to edit the query in the proxy so the duplicate item never happens in the first place. Is there an easy way to do this?