RDISCROWD-7705: Perform duplicate check using checksum / task payload

[dchhabda]

Issue number of the reported bug or feature request: #

Describe your changes
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Testing performed
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Additional context
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[coveralls]

Pull Request Test Coverage Report for Build 12587159603

Details

• 24 of 27 (88.89%) changed or added relevant lines in 5 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 94.104%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pybossa/task_creator_helper.py	6	7	85.71%
pybossa/repositories/task_repository.py	5	7	71.43%

Files with Coverage Reduction	New Missed Lines	%
pybossa/task_creator_helper.py	3	95.86%

Totals
Change from base Build 12471603829:	-0.03%
Covered Lines:	17493
Relevant Lines:	18589

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💛 - Coveralls