Ensure stub can be used multiple times on the same command

Ensures that the original bash function is preserved even if it is
stubbed multiple times before a restore.

stub.sh

@@ -69,10 +69,12 @@ stub_and_eval() {
 
   # If stubbing a function, store non-stubbed copy of it required for restore.
   if [ -n "$(command -v "$cmd")" ]; then
-    if [[ "$(type "$cmd" | head -1)" == *"is a function" ]]; then
-      local source="$(type "$cmd" | tail -n +2)"
-      source="${source/$cmd/non_stubbed_${cmd}}"
-      eval "$source"
+    if [ -z "$(command -v "non_stubbed_${cmd}")" ]; then
+      if [[ "$(type "$cmd" | head -1)" == *"is a function" ]]; then
+        local source="$(type "$cmd" | tail -n +2)"
+        source="${source/$cmd/non_stubbed_${cmd}}"
+        eval "$source"
+      fi
     fi
   fi
 

test/restore-test.sh

@@ -43,5 +43,17 @@ restore "my-name-is"
 assert "my-name-is Edward Elric" "My name is Edward Elric."
 
 
+# Stubbing the same function multiple times and then restoring it.
+my-name-is() { echo "My name is $@."; }
+stub "my-name-is"
+assert "my-name-is Edward Elric" ""
+stub "my-name-is" stdout
+assert "my-name-is Edward Elric" "my-name-is stub: Edward Elric"
+
+restore "my-name-is"
+assert "my-name-is Edward Elric" "My name is Edward Elric."
+
+
+
 # End of tests.
 assert_end "restore()"