deduplicate addresses.

pkg/discovery/cache/cache.go

@@ -38,12 +38,21 @@ func (c *Cache) Update(tgs []*targetgroup.Group) {
 
 // Addresses returns all the addresses from all target groups present in the Cache.
 func (c *Cache) Addresses() []string {
+	var addresses []string
+	var unique map[string]struct{}
+
 	c.Lock()
 	defer c.Unlock()
-	var addresses []string
+
+	unique = make(map[string]struct{})
 	for _, group := range c.tgs {
 		for _, target := range group.Targets {
-			addresses = append(addresses, string(target[model.AddressLabel]))
+			addr := string(target[model.AddressLabel])
+			if _, ok := unique[addr]; ok {
+				continue
+			}
+			addresses = append(addresses, addr)
+			unique[addr] = struct{}{}
 		}
 	}
 	return addresses

pkg/discovery/cache/cache_test.go

@@ -0,0 +1,39 @@
+package cache
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/prometheus/common/model"
+	"github.com/prometheus/prometheus/discovery/targetgroup"
+)
+
+func TestCacheAddresses(t *testing.T) {
+	tgs := make(map[string]*targetgroup.Group)
+	tgs["g1"] = &targetgroup.Group{
+		Targets: []model.LabelSet{
+			model.LabelSet{model.AddressLabel: "localhost:9090"},
+			model.LabelSet{model.AddressLabel: "localhost:9091"},
+			model.LabelSet{model.AddressLabel: "localhost:9092"},
+		},
+	}
+	tgs["g2"] = &targetgroup.Group{
+		Targets: []model.LabelSet{
+			model.LabelSet{model.AddressLabel: "localhost:9091"},
+			model.LabelSet{model.AddressLabel: "localhost:9092"},
+			model.LabelSet{model.AddressLabel: "localhost:9093"},
+		},
+	}
+
+	c := &Cache{tgs: tgs}
+
+	expected := []string{
+		"localhost:9090",
+		"localhost:9091",
+		"localhost:9092",
+		"localhost:9093",
+	}
+	if got := c.Addresses(); !reflect.DeepEqual(got, expected) {
+		t.Errorf("expected %v, want %v", got, expected)
+	}
+}