Implementation DISTINCT for window aggregate function: SUM

Hello PostgreSQL-development,

Oracle has implementation:

select id, amount, sum(DISTINCT amount) over () as total
from xx;

https://dbfiddle.uk/?rdbms=oracle_18&fiddle=8eeb60183ec9576ddb4b2c9f2874d09f

Why this is not possible in PG?
https://dbfiddle.uk/?rdbms=postgres_12&fiddle=97c05203af4c927ff9f206e164752767

Why Window-specific functions do not allow DISTINCT to be used within the function argument list.?
Which problems are exists?

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Best regards,
Eugen Konkov