TYPO3 Core

When our Customer is trying to Save an Entry in the Table "tx_3sfeusercreation_registerCustomization" via Backend-Module "List" there is an SQL-Error.
This is strange because this happens suddenly since today without any code changes and every time we try to save an Entry.

The Error says that the Primary-Key of "sys_refindex"-Tabele allready exists ("Duplicate entry '3b80e3c7f6e2d5bd6361ded6e0c13d99' for key 1").
When I check the Table via phpMyAdmin ("SELECT * FROM sys_refindex WHERE hash = '3b80e3c7f6e2d5bd6361ded6e0c13d99'"), I find one Entry.

I've also checked the Length of all "varchar"-Fields in "sys_refindex" and they are all long enough for the content.

I think Typo3 is trying to execute the Insert-Query twice, first time it works, second time it fails because the Primary-Key allready exists.

caller t3lib_DB::exec_INSERTquery

ERROR Duplicate entry '3b80e3c7f6e2d5bd6361ded6e0c13d99' for key 1

lastBuiltQuery
INSERT INTO sys_refindex
(
tablename,
recuid,
field,
flexpointer,
softref_key,
softref_id,
sorting,
deleted,
ref_table,
ref_uid,
ref_string,
hash
) VALUES (
'tx_3sfeusercreation_registerCustomization',
'176',
'subsite',
'',
'',
'',
'0',
'0',
'tx_3sfeuserextended_localization',
'101',
'',
'3b80e3c7f6e2d5bd6361ded6e0c13d99'
)

debug_backtrace SC_alt_doc->processData // t3lib_TCEmain->process_datamap // t3lib_TCEmain->updateDB // t3lib_TCEmain->updateRefIndex // t3lib_refindex->updateRefIndexTable // t3lib_DB->exec_INSERTquery // t3lib_DB->debug

Warning: Cannot modify header information - headers already sent by (output started at /srv/www/vhosts/typo3_src-4.0.12/t3lib/class.t3lib_db.php:1096) in /srv/www/vhosts/typo3_src-4.0.12/typo3/template.php on line 617
(issue imported from #M11423)