Fix #106, Reject duplicate msgid subscriptions WIP

[chillfig]

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• I reviewed the Contributing Guide.
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Describe the contribution

• Fixes Reject duplicate message ID in filter table validation #106

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System(s) tested on

• Hardware: [e.g. PC, SP0, MCP750]
• OS: [e.g. Ubuntu 18.04, RTEMS 4.11, VxWorks 6.9]
• Versions: [e.g. cFE 6.6, OSAL 4.2, PSP 1.3 for mcp750, any related apps or tools]

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Contributor Info - All information REQUIRED for consideration of pull request
Justin Figueroa, Vantage Systems Inc