fix shard assignment merge and mount errors

weed/storage/erasure_coding/shard_distribution.go

@@ -2,6 +2,7 @@ package erasure_coding
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -112,13 +113,31 @@ func DistributeEcShards(volumeID uint32, collection string, targets []*worker_pb
 			}
 		}
 
-		shardAssignment[target.Node] = assignedShards
+		existing := shardAssignment[target.Node]
+		if len(existing) == 0 {
+			shardAssignment[target.Node] = assignedShards
+			continue
+		}
+
+		seen := make(map[string]struct{}, len(existing))
+		for _, shard := range existing {
+			seen[shard] = struct{}{}
+		}
+		for _, shard := range assignedShards {
+			if _, ok := seen[shard]; ok {
+				continue
+			}
+			seen[shard] = struct{}{}
+			existing = append(existing, shard)
+		}
+		shardAssignment[target.Node] = existing
 	}
 
 	if len(shardAssignment) == 0 {
 		return nil, fmt.Errorf("no shard assignments found from planning phase")
 	}
 
+	var mountErrors []error
 	for destNode, assignedShards := range shardAssignment {
 		withFields(log, map[string]interface{}{
 			"destination":     destNode,
@@ -215,6 +234,7 @@ func MountEcShards(volumeID uint32, collection string, shardAssignment map[strin
 			})
 
 		if err != nil {
+			mountErrors = append(mountErrors, fmt.Errorf("mount %s shards %v: %w", destNode, shardIds, err))
 			withFields(log, map[string]interface{}{
 				"destination": destNode,
 				"shard_ids":   shardIds,
@@ -230,6 +250,9 @@ func MountEcShards(volumeID uint32, collection string, shardAssignment map[strin
 		}
 	}
 
+	if len(mountErrors) > 0 {
+		return errors.Join(mountErrors...)
+	}
 	return nil
 }