diff --git a/mcfg/cli.go b/mcfg/cli.go
index c6a3047..8aca3c2 100644
--- a/mcfg/cli.go
+++ b/mcfg/cli.go
@@ -122,31 +122,14 @@ func (cli SourceCLI) Parse(cfg *Cfg) ([]ParamValue, error) {
 
 func (cli SourceCLI) cliParamVals(cfg *Cfg) (map[string]ParamValue, error) {
 	m := map[string]ParamValue{}
-	for _, param := range cfg.Params {
+	for _, pv := range cfg.allParamValues() {
 		key := cliKeyPrefix
-		if !cfg.IsRoot() {
-			key += strings.Join(cfg.Path, cliKeyJoin) + cliKeyJoin
-		}
-		key += param.Name
-		m[key] = ParamValue{
-			Param: param,
-			Path:  cfg.Path,
+		if len(pv.Path) > 0 {
+			key += strings.Join(pv.Path, cliKeyJoin) + cliKeyJoin
 		}
+		key += pv.Param.Name
+		m[key] = pv
 	}
-
-	for _, child := range cfg.Children {
-		childM, err := cli.cliParamVals(child)
-		if err != nil {
-			return nil, err
-		}
-		for key, pv := range childM {
-			if _, ok := m[key]; ok {
-				return nil, fmt.Errorf("multiple params use the same CLI arg %q", key)
-			}
-			m[key] = pv
-		}
-	}
-
 	return m, nil
 }
 
diff --git a/mcfg/mcfg.go b/mcfg/mcfg.go
index 2507371..0d70100 100644
--- a/mcfg/mcfg.go
+++ b/mcfg/mcfg.go
@@ -62,19 +62,6 @@ func (h *Hook) Also(h2 Hook) {
 	}
 }
 
-// ParamValue describes a value for a parameter which has been parsed by a
-// Source
-type ParamValue struct {
-	Param
-	Path  []string // nil if root
-	Value json.RawMessage
-}
-
-// Source parses ParamValues out of a particular configuration source
-type Source interface {
-	Parse(*Cfg) ([]ParamValue, error)
-}
-
 // Cfg describes a set of configuration parameters and run-time behaviors.
 // Parameters are defined using the Param* methods, and run-time behaviors by
 // the Hook fields on this struct.
diff --git a/mcfg/source.go b/mcfg/source.go
new file mode 100644
index 0000000..185d9c9
--- /dev/null
+++ b/mcfg/source.go
@@ -0,0 +1,32 @@
+package mcfg
+
+import "encoding/json"
+
+// ParamValue describes a value for a parameter which has been parsed by a
+// Source
+type ParamValue struct {
+	Param
+	Path  []string // nil if root
+	Value json.RawMessage
+}
+
+func (cfg *Cfg) allParamValues() []ParamValue {
+	pvs := make([]ParamValue, 0, len(cfg.Params))
+	for _, param := range cfg.Params {
+		pvs = append(pvs, ParamValue{
+			Param: param,
+			Path:  cfg.Path,
+		})
+	}
+
+	for _, child := range cfg.Children {
+		pvs = append(pvs, child.allParamValues()...)
+	}
+	return pvs
+}
+
+// Source parses ParamValues out of a particular configuration source. The
+// returned []ParamValue may contain duplicates of the same Param's value.
+type Source interface {
+	Parse(*Cfg) ([]ParamValue, error)
+}