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This question was asked by various individuals on Telegram.

The spec (BOLT 2) allows for multiple channels between the same two peers:

Note that multiple channels can operate in parallel, as all channel messages are identified by either a temporary_channel_id (before the funding transaction is created) or a channel_id (derived from the funding transaction).

The temporary_channel_id is unique only within a single connection hence implying the possibility of multiple channels per peer/connection.

As far as c-lightning goes, I get the following error when trying to fund a second channel with the same peer:

Peer already CHANNELD_NORMAL

Why doesn't c-lightning follow the spec here?

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This was answered by Christian Decker on Telegram.

Just because the specification allows something doesn't mean every implementation has to implement it and support it. In c-lightning's case it is both an architectural choice (multiplexing multiple channels onto a TCP connection is not trivial) as well as a conviction (peer diversity beats doubling down on a single peer with multiple channels, and avoids the creation of single points of failure). After all dual funding and splicing give the same abilities as channel multiplexing with a reduced onchain footprint as well as less complexity and less load on the gossip network.

Rene Pickhardt added in the comments below:

Parallel channels might also be a challenge for payment flows. First the computation becomes more expensive and second the assigned liquidity is artificially split between two separate channels. Thus one should always prefer dual funding, splicing and rebalancing over parallel channels

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    I want to add that parallel channels might also be a challenge for payment flows. First the computation becomes more expensive and second the assigned liquidity is artificially split between two separate channels. Thus one should always prefer dual funding, splicing and rebalancing over parallel channels Nov 1, 2021 at 11:50

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