'It never rains but [what] it pours' is a proverb with the meaning you first state. It's almost always used as a metaphor (ie not in the literal sense; rain speaks of misfortune).

Both literal usages and figurative ones can be used antiphrastically (ie with an opposite meaning to the straightforward one).

• 'What delightful weather.'
• 'Aren't you bright!'

But a double entendre is supposed to be readable in two ways at the same time, not in just one out of two distinct senses. And often, reading between the lines leads to an intended amusing sexual innuendo.

• If I said you had a beautiful body would you hold it against me? (lyrics)

But not always:

• Children make delicious snacks (newspaper headline)

• Criminals Receive 9 Months in the Violin Case (newspaper headline)

• New York Jets run out of fuel (witty headline)

'It never rains but [what] it pours' is a proverb with the meaning you first state. It's almost always used as a metaphor (ie not in the literal sense; rain speaks of misfortune).

Both literal usages and figurative ones can be used antiphrastically (ie with an opposite meaning to the straightforward one).

• 'What delightful weather.'
• 'Aren't you bright!'

But a double entendre is supposed to be readable in two ways at the same time, not in just one out of two distinct senses. And often, reading between the lines leads to an intended amusing sexual innuendo.

• If I said you had a beautiful body would you hold it against me? (lyrics)

But not always:

• Children make delicious snacks (newspaper headline)

• Criminals Receive 9 Months in the Violin Case (newspaper headline)

• New York Jets run out of Fuel (witty headline)