Merging two already linked tickets can prevent the tickets being linked after the merge.

[svenoe]

During ticket merge first all existing links should be transfered from the merged ticket into the "merged into" ticket, and then a link between both tickets should be created:

otobo/Kernel/System/Ticket.pm

Lines 6563 to 6571 in 9b6f633

	# transfer all linked objects to new ticket
	$Self->TicketMergeLinkedObjects(
	MergeTicketID => $Param{MergeTicketID},
	MainTicketID => $Param{MainTicketID},
	UserID => $Param{UserID},
	);
	# link tickets
	$Kernel::OM->Get('Kernel::System::LinkObject')->LinkAdd(

This seems to fail at times, which I assume is due to TicketMergeLinkedObjects() removing the previously existing link, but not deleting the cache:

otobo/Kernel/System/Ticket.pm

Lines 6824 to 6838 in 9b6f633

	# Delete all links between tickets where source and target object are the same.
	$DBObject->Do(
	SQL => '
	DELETE FROM link_relation
	WHERE source_object_id = ?
	AND target_object_id = ?
	AND source_key = target_key
	',
	Bind => [
	\$TicketObjectID,
	\$TicketObjectID,
	],
	);
	return 1;

Which, if the link also was of the parent child type, will let LinkAdd() run into this error:

otobo/Kernel/System/LinkObject.pm

Line 519 in 9b6f633

Message => 'Another Link already exists within the same type group!',

Please confirm and fix by probably just clearing the cached links in TicketMergeLinkedObjects().