Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.8.3 darwin/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"

What did you do?

Unmarshal to existing map[string]interface{} that is hidden behind an interface{}:
https://play.golang.org/p/kgaRznDOwb

var test interface{}
if err := json.Unmarshal([]byte(`{"A": "B"}`), &test); err != nil {
	panic(err)
}
fmt.Printf("First:\n %+v\n", test)

if err := json.Unmarshal([]byte(`{"C": "D"}`), &test); err != nil {
	panic(err)
}
fmt.Printf("Second:\n %+v\n", test)

What did you expect to see?

First:
map[A:B]
Second:
map[A:B C:D]

According to docs:

To unmarshal JSON into an interface value, Unmarshal stores one of these in the interface value:
...
map[string]interface{}, for JSON objects

To unmarshal a JSON object into a map, Unmarshal first establishes a map to use. If the map is nil, Unmarshal allocates a new map. Otherwise Unmarshal reuses the existing map, keeping existing entries.

I expect unmarshal to reuse the existing map, instead of replacing it.

What did you see instead?

First:
map[A:B]
Second:
map[C:D]