I have a project that invokes more than one Ant build step during the execution of a job.
I happened to need to insert multiple properties in the second build step and in order to do that I had to change to the textarea mode, that's when I noticed that the javascript wasn't working properly due to the fact that some form elements are being created with repeated ids.

Next thing I tried was to reproduced in a clean Jenkins environment, so I just downloaded the WAR and unzipped a clean version of Tomcat and was able to reproduce the bug by doing this:

• New job -> Configure
• Add build step #1
• Add build step #2
• Click in the expand button of build step #2 "Targets" field
• Notice that the "Targets" field of #1 will be changed to a textarea instead of #2