itest: add test for duplicate address send

This test demonstrates that we get the correct error when we attempt to
send to the same address twice in the same transaction. This also shows
that we can re-try normally if a send fails before it was written to
disk.

itest/send_test.go

@@ -17,6 +17,7 @@ import (
 	"github.com/lightninglabs/taproot-assets/taprpc/mintrpc"
 	"github.com/lightninglabs/taproot-assets/taprpc/tapdevrpc"
 	unirpc "github.com/lightninglabs/taproot-assets/taprpc/universerpc"
+	"github.com/lightninglabs/taproot-assets/tapsend"
 	"github.com/lightningnetwork/lnd/lntest/wait"
 	"github.com/stretchr/testify/require"
 )
@@ -102,6 +103,16 @@ func testBasicSendUnidirectional(t *harnessTest) {
 	})
 	require.NoError(t.t, err)
 
+	// Before we start sending, we test that we aren't allowed to send to
+	// the same address more than once within the same transfer.
+	_, err = t.tapd.SendAsset(ctxb, &taprpc.SendAssetRequest{
+		TapAddrs: []string{
+			bobAddr.Encoded,
+			bobAddr.Encoded,
+		},
+	})
+	require.ErrorContains(t.t, err, tapsend.ErrDuplicateScriptKeys.Error())
+
 	for i := 0; i < numSends; i++ {
 		t.t.Logf("Performing send procedure: %d", i)