bugfix: prevent duplicate connections in pool

An user can specify duplicate addresses for a ConnectionPool. We
cannot support multiple connections to the same address due to
the ConnectionPool.GetPoolInfo() implementation without breaking
backward compatibility. So we need to skip duplicates.

Closes #208

connection_pool/connection_pool.go

@@ -97,7 +97,7 @@ func ConnectWithOpts(addrs []string, connOpts tarantool.Opts, opts OptsPool) (co
 	anyPool := NewEmptyRoundRobin(size)
 
 	connPool = &ConnectionPool{
-		addrs:    make([]string, len(addrs)),
+		addrs:    make([]string, 0, len(addrs)),
 		connOpts: connOpts,
 		opts:     opts,
 		notify:   notify,
@@ -107,7 +107,14 @@ func ConnectWithOpts(addrs []string, connOpts tarantool.Opts, opts OptsPool) (co
 		roPool:   roPool,
 		anyPool:  anyPool,
 	}
-	copy(connPool.addrs, addrs)
+
+	m := make(map[string]bool)
+	for _, addr := range addrs {
+		if _, ok := m[addr]; !ok {
+			m[addr] = true
+			connPool.addrs = append(connPool.addrs, addr)
+		}
+	}
 
 	somebodyAlive := connPool.fillPools()
 	if !somebodyAlive {

connection_pool/connection_pool_test.go

@@ -80,6 +80,31 @@ func TestConnSuccessfully(t *testing.T) {
 	require.Nil(t, err)
 }
 
+func TestConnSuccessfullyDuplicates(t *testing.T) {
+	server := servers[0]
+	connPool, err := connection_pool.Connect([]string{server, server, server, server}, connOpts)
+	require.Nilf(t, err, "failed to connect")
+	require.NotNilf(t, connPool, "conn is nil after Connect")
+
+	defer connPool.Close()
+
+	args := test_helpers.CheckStatusesArgs{
+		ConnPool:           connPool,
+		Mode:               connection_pool.ANY,
+		Servers:            []string{server},
+		ExpectedPoolStatus: true,
+		ExpectedStatuses: map[string]bool{
+			server: true,
+		},
+	}
+
+	err = test_helpers.CheckPoolStatuses(args)
+	require.Nil(t, err)
+
+	addrs := connPool.GetAddrs()
+	require.Equalf(t, []string{server}, addrs, "should be only one address")
+}
+
 func TestReconnect(t *testing.T) {
 	server := servers[0]