How to copy documents from one collection into another collection

This Meteor code line FormDataCol.insert(temp1); puts the document temp1 in the collection FormData which was verified at the mongo terminal

meteor:PRIMARY> db.formData.find({}).pretty();
{
	"_id" : "zTKkCrGZhBbeL2kvS",
	"element" : "input",
	"action" : "order",
	"name" : "plate",
	"placeholder" : "Enter plate code",
	"valid" : "^[0-9a-zA-Z]{1,6}$",
	"value" : "somevalue",
	"class" : "wrongInput"
}
meteor:PRIMARY> 

How can I add another document temp2 which has a different _id into the same collection with out getting the familiar key duplicate error, so I get the following output?

Both documents have been copied from other collection and more fields/values have been added to them so that “temp” is the “new” document that need to be “inserted”, how ever the _d is not important to keep the same. Thanks

    meteor:PRIMARY> db.formData.find({}).pretty();
    {
    	"_id" : "zTKkCrGZhBbeL2kvS",
    	"element" : "input",
    	"action" : "order",
    	"name" : "plate",
    	"placeholder" : "Enter plate code",
    	"valid" : "^[0-9a-zA-Z]{1,6}$",
    	"value" : "somevalue",
    	"class" : "wrongInput"
    }
{
	"_id" : "h5nxkAM5hBihS8Zeb",
	"element" : "radio",
	"action" : "cooke",
	"name" : "x-large"
}