Fixed multiple generation of event with the same error in Meta Data Point

[Limraj]

To Reproduce
Steps to reproduce the behavior:

1. Login as admin;
2. Created Virtual Data Source as vds_1 (enabled, update: 5s) with point as vdp_1 (enabled, and default settings)
3. Created Meta Data Source as mds_1 (enabled) with point as mdp_1 (enabled) and add to context:

• vds_1 - vdp_1 as p1;
  and set script: return p2.value;;
  and click on 'Save';

4. Go to 'Alarms' then generate event every 5s-> error;