Fix repeated execution

[vodovozovaliza]

This PR closes issue #917.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (53515a6) 100.00% compared to head (1674430) 100.00%.

❗ Current head 1674430 differs from pull request most recent head 63d42a6. Consider uploading reports for the commit 63d42a6 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #918   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           48        48           
  Lines         6593      6586    -7     
=========================================
- Hits          6593      6586    -7     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/qibo/backends/numpy.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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