I check the form on the page through js (all fields must be filled out, only Latin letters, etc.). The question is: how much is this check safe and is it worth duplicating it on the server side?

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    10000000000% must be duplicated! It isn’t done differently - Alexey Shimansky

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There is a golden rule - never trust a client.
Adhering to it, life will improve :)

So, yes, duplication of verification is necessary, on the client, rather, the recommendation and voice of the rules on how to deliver the data, and not the actual verification, since anyone with a sniffer and brain can forge a request outside of any client checks.