Add errors for redundant definitions.

[alanmcruickshank]

If we attempt to redefine a dialect element identically, this will now throw an error. It's another development safety net to catch some examples I've already removed.

I've added it with no cover because dialects are now clean, but tested it by adding some redundant definitions in and checking they throw errors.

[codecov]

Codecov Report

Merging #3626 (0b58cd3) into main (6b31de4) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #3626   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          175       175           
  Lines        13261     13264    +3     
=========================================
+ Hits         13261     13264    +3     

Impacted Files	Coverage Δ
src/sqlfluff/dialects/dialect_redshift.py	100.00% <ø> (ø)
src/sqlfluff/dialects/dialect_sparksql.py	100.00% <ø> (ø)
src/sqlfluff/core/dialects/base.py	100.00% <100.00%> (ø)
src/sqlfluff/core/parser/grammar/base.py	100.00% <100.00%> (ø)
src/sqlfluff/core/parser/grammar/delimited.py	100.00% <100.00%> (ø)

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