Use unique versions in staggered versioner (fixes #1063)

internal/versioner/util.go

@@ -18,6 +18,7 @@ package versioner
 import (
 	"path/filepath"
 	"regexp"
+	"sort"
 )
 
 // Inserts ~tag just before the extension of the filename.
@@ -40,3 +41,17 @@ func filenameTag(path string) string {
 	}
 	return match[1]
 }
+
+func uniqueSortedStrings(strings []string) []string {
+	seen := make(map[string]struct{}, len(strings))
+	unique := make([]string, 0, len(strings))
+	for _, str := range strings {
+		_, ok := seen[str]
+		if !ok {
+			seen[str] = struct{}{}
+			unique = append(unique, str)
+		}
+	}
+	sort.Strings(unique)
+	return unique
+}