Submitting "On Behalf Of" Contributions generates "DB error : already exists" (Joomla! 2.5.7 + civicrm-4.2.1-joomla-alt)

Problem : DB Error : Already exists - on "Contribution Page" Submission

Background :

"User" registers on site via a CiviCRM "Contribution Page".

That "Contribution Page" includes :

• "Membership" tab > a "Membership" = "Required"
• "Profile" tab > "New Individual" Profile (with "Advanced Settings" > "Joomla user account registration option?" = "Account creation required")

• Result = CiviCRM & Joomla Reg OK. Joomla User = "Registered"

"User" then attempts to make an "On Behalf Of" contribution via another "Contribution Page".

That "Contribution Page" includes :

• "Title" tab > "Allow individuals to contribute .... " = "Ticked" > "Organization Profile" = "New Organization" > "Check 'Required' to force ... organization" > value = "Optional".
• "Amounts" tab > "Dummy Payment Processor" > 1 x Amount
• "Membership" tab > a "Membership" = "Required"

• Result : "User" clicks > "Confirm Contribution" button > "Confirmation Page" > "Make Contribution" button > generates "DB Error : already exists"

WHAT "ALREADY EXISTS" ?

No matter what combination of "Organization" Profiles & values specified on that "Contribution Page" submission results in "DB Error : already exists"

When reviewed in "Admin", CiviCRM has created a new "Contact" for that "New Organization" & the "Users" details have been updated to included "Billing Address" info generated by completing the Credit Card payment area.

Completely deleting both the "User" & the "New Organization" from both Joomla & CiviCRM prior to re-testing does not effect the outcome when the test is repeated.

I have created a site to demo the problem. Site created on new PHP 5.3.17 server with Joomla 2.5.7 & CiviCRM 4.2.1

The site has only one "User", "Super Admin". admin login is Username = admin, p/w = admin (http://test.xitranet.com/administrator)

Site : http://test.xitranet.com

When you view site, on main menu find "CiviCRM + Joomla Membership". Completing that form creates a Joomla Registered User with a CiviCRM "Contact" record.

Now Login with the Username & P/W you created & new options with be available on main menu > "On Behalf Of Test 01-04" - each test relates to a CiviCRM contribution page.

P.S. When you access Joomla Admin you wont find CiviCRM in "Components" because all version of CiviCRM for servers that don't have "built-in unzip functionality" (eg : civicrm-4.2.1-joomla-alt) fail on install because they contain code relating to "Server unzip functionality".

To get around that problem you have to deleted the first few lines of ".../tmp/com_civicrm/admin/configure.php" prior to install via Joomla "Extension Manager" to remove references to "server unzip functionality". As a result of this mucking around CiviCRM does install but does not appear in the "Components" list and can only be accessed from the Joomla "Control Panel").