This article describes the symptom of firewall filter counters displaying twice the expected value under certain conditions.
When Filter-Based Forwarding (FBF) or Policy-Based Routing (PBR) is configured with firewall filter counters on platforms QFX5120, EX4650, EX4100, EX4400, the CLI output for FBF/PBR filter statistics shows double the actual packet count.
This is just a cosmetic issue and has no functional impact.
There is no workaround, so you may need to upgrade to a fixed release or consider this when reading counters along with FBF or PBR.
https://prsearch.juniper.net/PR1863813 addresses this issue.
Please refer to the PR for fixed release information.