A defined user parameter in Zabbix agent is returning values twice for every interval specified. For a relatively long running check, you'll see that each zabbix agent process which is responsible for processing active checks spawns a process to perform this active check.

How to reproduce:
Setup a UserParameter and configure it in server as an active check. Check latest data to see that values arrive twice at each interval.

Below a verification from the process listing (look at the parent process of the executed script:
–
zabbix 20201 1 0 08:48 ? 00:00:00 /usr/sbin/zabbix_agentd
zabbix 20204 20201 0 08:48 ? 00:00:00 /usr/sbin/zabbix_agentd: collector [idle 1 sec]
zabbix 20205 20201 0 08:48 ? 00:00:00 /usr/sbin/zabbix_agentd: listener #1 [waiting for connection]
zabbix 20206 20201 0 08:48 ? 00:00:00 /usr/sbin/zabbix_agentd: listener #2 [waiting for connection]
zabbix 20207 20201 0 08:48 ? 00:00:00 /usr/sbin/zabbix_agentd: listener #3 [waiting for connection]
zabbix 20208 20201 0 08:48 ? 00:00:00 /usr/sbin/zabbix_agentd: active checks #1 [processing active checks]
zabbix 20209 20201 0 08:48 ? 00:00:00 /usr/sbin/zabbix_agentd: active checks #2 [processing active checks]
zabbix 20238 20208 0 08:48 ? 00:00:00 sh -c /usr/local/bin/mailqr -m -t -i 8 -c 3
zabbix 20239 20238 0 08:48 ? 00:00:00 /bin/bash /usr/local/bin/mailqr -m -t -i 8 -c 3
zabbix 20274 20209 0 08:48 ? 00:00:00 sh -c /usr/local/bin/mailqr -m -t -i 8 -c 3
zabbix 20275 20274 0 08:48 ? 00:00:00 /bin/bash /usr/local/bin/mailqr -m -t -i 8 -c 3