xml helper allows multiple instances of name to be merge in final dest xml

This some what related to CB-10961

Create new app
Add multiple name tags in toplevel config.xml
<name>app1</name>
<name>app2</name>
<platform name="ios">
<name>app3</name>
</platform>

cordova platform add ios
cordova prepare

Open platforms/ios/app3/config.xml

There are multiple instances of name at top level
<name>app3</name>

<name>app1</name>
<name>app2</name>

The mergeXml function should be updated to avoid this and only allow one final name tag added to top level dest.
https://github.com/apache/cordova-lib/blob/master/cordova-common/src/util/xml-helpers.js#L198