Panic if trying to register an func with the same name

Signed-off-by: Michael Crosby <michael@docker.com>

reexec/reexec.go

@@ -1,11 +1,18 @@
 package reexec
 
-import "os"
+import (
+	"fmt"
+	"os"
+)
 
 var registeredInitializers = make(map[string]func())
 
 // Register adds an initialization func under the specified name
 func Register(name string, initializer func()) {
+	if _, exists := registeredInitializers[name]; exists {
+		panic(fmt.Sprintf("reexec func already registred under name %q", name))
+	}
+
 	registeredInitializers[name] = initializer
 }