16987: remove dups in Intersection at instantiation

[smichr]

References to other Issues or PRs

Fixes #16987 without fully addressing the fact that a different wrong value is being returned from Intersection({1}, {x}). The value of {1} is being returned by simplify_intersection. (But this is only true if x is also 1.)

Brief description of what is fixed or changed

Intersection simplification depends on the presence of duplicates as shown in the issue. In addition, there is some indeterminancy in the processing because the arguments are not sorted.

This PR fixes both those issue.

Other comments

Release Notes

• sets
  • processing of arguments of an intersection is more canonical

[sympy-bot]

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• sets
  • processing of arguments of an intersection is more canonical (#16988 by @smichr)

This will be added to https://github.com/sympy/sympy/wiki/Release-Notes-for-1.5.

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#### References to other Issues or PRs
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Fixes #16987 without fully addressing the fact that a different wrong value is being returned from `Intersection({1}, {x})`. The value of `{1}` is being returned by `simplify_intersection`. (But this is only true if `x` is also `1`.)

#### Brief description of what is fixed or changed

Intersection simplification depends on the presence of duplicates as shown in the issue. In addition, there is some indeterminancy in the processing because the arguments are not sorted.

This PR fixes both those issue.

#### Other comments


#### Release Notes

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<!-- BEGIN RELEASE NOTES -->
- sets
    - processing of arguments of an intersection is more canonical
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #16988 into master will increase coverage by 0.013%.
The diff coverage is 100%.

@@              Coverage Diff              @@
##            master    #16988       +/-   ##
=============================================
+ Coverage   73.956%   73.969%   +0.013%     
=============================================
  Files          620       620               
  Lines       160517    160517               
  Branches     37656     37656               
=============================================
+ Hits        118713    118734       +21     
+ Misses       36327     36299       -28     
- Partials      5477      5484        +7