No second bond is necessary.
The existing bond should be connected not more than 600mm from the meter on the dicharge side, to the main electrical earth connection at the incoming power supply. It is recommended that this bond cable should 6mm Green/Yellow. It is also recommended that the cross bonding continues to pickup the incoming water supply pipework.
If you incoming gas supply is bonded at your side of the meter within 600mm of it entering building and back at MET (main earth terminal) at the CU, that's as it should be.
Your new boiler will have a plate underneath that connects all pipes to that so no need to additional bond or supplementary bond the boiler.
The only case I'd consider additional bonding, would be if the mains gas entered the building at more than one point.
i.e meter in cabinet outside wall, supply coming in downstairs to cooker and supply going up external wall to boiler upstairs.
But in theroy it should still be earthed back at original point of enterary downstairs.
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