Clarify script.addPreloadScript with duplicate context (#620)

Author: Lightning00Blade

index.bs

@@ -8298,16 +8298,20 @@ The [=remote end steps=] given |session| and |command parameters| are:
 1. Let |arguments| be the <code>arguments</code> field of |command
    parameters| if present, or an empty [=/list=] otherwise.
 
-1. Let the |contexts| be the value of the <code>contexts</code> field of |command parameters| if it is present or null if it isn’t.
+1. Let |contexts| be null.
 
-1. If |contexts| is not null:
+1. If the <code>contexts</code> field of |command parameters| is present:
 
-   1. For each |context id| of |contexts|:
+   1. Set |contexts| to an empty [=/set=].
+
+   1. For each |context id| of |command parameters|["<code>contexts</code>"]
 
       1. Let |context| be the result of [=trying=] to [=get a browsing context=] with |context id|.
 
       1. If |context| is not a [=top-level browsing context=], return [=error=] with [=error code=] [=invalid argument=].
 
+      1. Append |context| to |contexts|.
+
 1. Let |sandbox| be the value of the "<code>sandbox</code>" field in |command
    parameters|, if present, or null otherwise.