Avoid storing duplicate functions/methods in CallbackContainer

[astrofrog]

Another of the items in #45

[codecov]

Codecov Report

Attention: Patch coverage is 97.75281% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.88%. Comparing base (8f8eb9a) to head (0fa96a0).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
echo/tests/test_callback_container.py	97.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #48      +/-   ##
==========================================
+ Coverage   96.77%   96.88%   +0.11%     
==========================================
  Files          18       19       +1     
  Lines        2356     2441      +85     
==========================================
+ Hits         2280     2365      +85     
  Misses         76       76              

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