I've seen this on DSA campaigns (maybe a DSA bug).
Adwords Account ID (client customer ID):
5845103608
Fetching this report through the API:
{'dateRangeType': 'CUSTOM_DATE',
'downloadFormat': 'CSV',
'reportName': 'Custom CAMPAIGN_PERFORMANCE_REPORT',
'reportType': 'CAMPAIGN_PERFORMANCE_REPORT',
'selector': {'dateRange': {'max': '20170413', 'min': '20170411'},
'fields': ['Date', 'CampaignId', 'BounceRate'],
'predicates': []}}
Excerpt of the received CSV:
['2017-04-12,347606525,0.00%\n',
'2017-04-11,347606525,100.00%\n',
'2017-04-11,347606525,47.03%\n',
'2017-04-12,347606525,44.35%\n',
'2017-04-13,347606525,40.66%\n']
As you can see, some dates are present twice, with different bounce rates.
When adding another columns like Clicks or ClickAssistedConversions, Cost, the report is re-aggregated (example: the cost is doubled).