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I guess this means “two precautions are better than one” or “better safe than sorry”.
What does “valent” mean? It doesn't derive from être, but seems to be acting like “sont” in this case?
From the infinitive “valoir” (TLFi), meaning “to be equivalent to” or “to be worth”.
“Valoir mieux que” means “to be better than” or “to be worth more than”.
The form “valent” is the third person plural of the present tense.
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