`PFMERGE` wrong behavior

[Niennienzz]

Describe the Bug

• PFMERGE accumulates the cardinality of destination.
• If the destination HyperLogLog exists, it should be treated as one of the sources.
• But since HyperLogLog counts unique values, the cardinality should not accumulate (see examples below).

To Reproduce

$> PFADD hll_01 1 2 3
(integer) 1
$> PFADD hll_02 4 5
(integer) 1
$> PFMERGE destination hll_01 hll_02
OK
$> PFCOUNT destination
(integer) 5

# No new unique member added.
# Repeat the previous two commands.
$> PFMERGE destination hll_01 hll_02
OK
$> PFCOUNT destination
(integer) 10                           # This is wrong.
$> PFMERGE destination hll_01 hll_02
OK
$> PFCOUNT destination
(integer) 15                           # This is wrong.

Expected Behavior

• There's no new unique member added in hll_01, hll_02, and destination.
• The merged destination HyperLogLog should have the same value onwards.

$> PFADD hll_01 1 2 3
(integer) 1
$> PFADD hll_02 4 5
(integer) 1
$> PFMERGE destination hll_01 hll_02
OK
$> PFCOUNT destination
(integer) 5

# No new unique member added.
# Repeat the previous two commands.
$> PFMERGE destination hll_01 hll_02
OK
$> PFCOUNT destination
(integer) 5
$> PFMERGE destination hll_01 hll_02
OK
$> PFCOUNT destination
(integer) 5

Environment

• OS: MacOS
• Kernel: Darwin Kernel Version 23.5.0: Wed May 1 20:14:38 PDT 2024; root:xnu-10063.121.3~5/RELEASE_ARM64_T6020 arm64
• Containerized: Docker
• Dragonfly Version: v1.27.2

[kostasrim]

Thank you @Niennienzz !