SSRS nested parent/child row group shows double first line

[Bart_Bertels]

Dear,

I want to group data on 2 levels (parent-child)

Level1: Analysisname

Level2: AnalysisUnit

IMy first step is still 'ok' : 

However, when moving the Parent above the child colums, I always get a first line which is double:

I can provide you with the dataset in Excel format and the 2 rdl report files.

Can someone please help me on this one. It is driving me CRAZY !!!!

AnalysisResultKey	AnalysisKey	AliasKey	AliasResultKey	AliasNumber	QTY AliasNumber	AnalysisName	AnalysisCommentLineNr	AnalysisUnit	AnalysisLimit
1_35814	_35814	_35814	1_35814	35814	3	TSH	NULL	NULL
2_35814	_35814	_35814	2_35814	35814	3	NULL	NULL	NULL
3_35814	_35814	_35814	3_35814	35814	3	NULL	3	3. Trimenon	0.3-3.0
3_35814	_35814	_35814	3_35814	35814	3	NULL	2	2. Trimenon	0.2-3.0
3_35814	_35814	_35814	3_35814	35814	3	NULL	1	1. Trimenon	0.1-2.5
4_35814	_35814	_35814	4_35814	35814	3	NULL	3	3. Trimenon	0.3-3.0
4_35814	_35814	_35814	4_35814	35814	3	NULL	2	2. Trimenon	0.2-3.0
4_35814	_35814	_35814	4_35814	35814	3	NULL	1	1. Trimenon	0.1-2.5
5_35814	_35814	_35814	5_35814	35814	3	NULL	3	3. Trimenon	0.3-3.0
5_35814	_35814	_35814	5_35814	35814	3	NULL	2	2. Trimenon	0.2-3.0
5_35814	_35814	_35814	5_35814	35814	3	NULL	1	1. Trimenon	0.1-2.5

[#2]

Adding OPTION (MAXDOP 1) at the end of the dataset query, solves the issue. However, as soon as I expand the report template with the other data I need, the issue is there again. Anyone had the same behaviour, i.e. dataset with the OPTION (MAXDOP 1) show different output in the report compared to dataset without OPTION (MAXDOP 1) ?