remove duplicates in Dependencies

duplicate entries could end up in "depends_on" in the state, which could
possible lead to erroneous state comparisons. Remove them when walking
the graph, and remove existing duplicates when pruning the state.

terraform/node_resource_abstract.go

@@ -102,7 +102,7 @@ func (n *NodeAbstractResource) References() []string {
 			}
 		}
 
-		return result
+		return uniqueStrings(result)
 	}
 
 	// If we have state, that is our next source

terraform/state.go

@@ -1163,6 +1163,8 @@ func (m *ModuleState) prune() {
 			delete(m.Outputs, k)
 		}
 	}
+
+	m.Dependencies = uniqueStrings(m.Dependencies)
 }
 
 func (m *ModuleState) sort() {
@@ -1526,8 +1528,9 @@ func (s *ResourceState) prune() {
 			i--
 		}
 	}
-
 	s.Deposed = s.Deposed[:n]
+
+	s.Dependencies = uniqueStrings(s.Dependencies)
 }
 
 func (s *ResourceState) sort() {

terraform/util.go

@@ -1,6 +1,7 @@
 package terraform
 
 import (
+	"sort"
 	"strings"
 )
 
@@ -73,3 +74,20 @@ func strSliceContains(haystack []string, needle string) bool {
 	}
 	return false
 }
+
+// deduplicate a slice of strings
+func uniqueStrings(s []string) []string {
+	if len(s) < 2 {
+		return s
+	}
+
+	sort.Strings(s)
+	result := make([]string, 1, len(s))
+	result[0] = s[0]
+	for i := 1; i < len(s); i++ {
+		if s[i] != result[len(result)-1] {
+			result = append(result, s[i])
+		}
+	}
+	return result
+}