$merge results in duplicates

The $merge operator is creating duplicates that do not happen when the same query is executed via MongoDB console. I have a simple collection called “details”:

{
	"_id" : ObjectId("5e18c9a02a24ce9706adc71b"),
	"x" : 5,
	"name" : "kate4"
},
{
	"_id" : ObjectId("5e18c9a02a24ce9706adc71a"),
	"x" : 4,
	"name" : "kate3"
},
{
	"_id" : ObjectId("5e18c9a02a24ce9706adc719"),
	"x" : 3,
	"name" : "kate2"
},
{
	"_id" : ObjectId("5e18c9a02a24ce9706adc718"),
	"x" : 2,
	"name" : "kate1"
},
{
	"_id" : ObjectId("5e18c9a02a24ce9706adc717"),
	"x" : 1,
	"name" : "smith"
}

I then run this query:

db.details.aggregate([
{"$group":{
    _id:{},
    "sum":{$sum:"$x"} 
  }
},
{"$project":{"_id":0,"sum":1}}
{"$merge":
  {"into": "summary"}
}    

])

BUT, every time it runs it creates two docs in the summary table. When I run the same query through MongoDB console, it creates one document as expected. Please investigate.