Add support for multiple alias and remove duplication of SSL certificates (#4472)

internal/ingress/annotations/alias/main.go

@@ -17,7 +17,11 @@ limitations under the License.
 package alias
 
 import (
+	"sort"
+	"strings"
+
 	networking "k8s.io/api/networking/v1beta1"
+	"k8s.io/apimachinery/pkg/util/sets"
 
 	"k8s.io/ingress-nginx/internal/ingress/annotations/parser"
 	"k8s.io/ingress-nginx/internal/ingress/resolver"
@@ -35,5 +39,25 @@ func NewParser(r resolver.Resolver) parser.IngressAnnotation {
 // Parse parses the annotations contained in the ingress rule
 // used to add an alias to the provided hosts
 func (a alias) Parse(ing *networking.Ingress) (interface{}, error) {
-	return parser.GetStringAnnotation("server-alias", ing)
+	val, err := parser.GetStringAnnotation("server-alias", ing)
+	if err != nil {
+		return []string{}, err
+	}
+
+	aliases := sets.NewString()
+	for _, alias := range strings.Split(val, ",") {
+		alias = strings.TrimSpace(alias)
+		if len(alias) == 0 {
+			continue
+		}
+
+		if !aliases.Has(alias) {
+			aliases.Insert(alias)
+		}
+	}
+
+	l := aliases.List()
+	sort.Strings(l)
+
+	return l, nil
 }