No, loadfile doesn't overwrite properties. PR 46440.

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@731895 13f79535-47bb-0310-9956-ffa450edef68

docs/manual/CoreTasks/loadresource.html

@@ -37,6 +37,9 @@ element resource collections. If the resource content is empty (maybe after
 processing a filterchain) the property is not set.
 </p>
 
+<p>Since properties are immutable, the task will not change the value
+  of an existing property.</p>
+
 <h3>Parameters</h3>
 <table border="1" cellpadding="2" cellspacing="0">
   <tr>