I had a problem with a duplicate tab and we have a 30K+ line template file which I didn't really want to look through so I wrote a quick, dirty, little Perl script to find duplicate tabs. There are probably better ways of doing this, but it's 3AM and this was my first thought.  Change "exportedtemplate.template" to your template's filename. After you find the problem, just search the template file for "<name>TABNAME</name>" to find the duplicates and you should be able to remove it manually or comment it out with <!-- -->

#!/usr/bin/perl

use XML::XPath;
use XML::XPath::XMLParser;

my $xp = XML::XPath->new(filename => 'exportedtemplate.template');

my $nodeset = $xp->find('/portal/tabs/tab'); # find all paragraphs

%tabs = ();

foreach my $node ($nodeset->get_nodelist) {
$name = $node->find('name')->string_value;
$parent = $node->find('parent')->string_value;

if($tabs{"$name $parent"}){
    print "DUPLICATE: TABNAME=$name PARENT=$parent\n";
}

$tabs{"$name $parent"} = 1;
}

Issue Resolved for me

I tried upgrading from 5.2.3 to 5.4.0 and it did not resolve the issue. Finally, I decided to rebuild the DotNetNuke Library source to add more information than "Tab exists". Specifically, I added - Throw New TabExistsException(iTabID, "Tab Exists" + "[iTabID:" + iTabID.ToString() + "], [TabPath:" + objTab.TabPath + "]") in TabController.vb line 90.

This will tell you exactly what is wrong with your template file (i.e. which tab it won't accept). Open your template file with a XML editor and you should be able to just remove the TAB node that is not playing well with your template.

Hopes this helps anyone else with the same issue.