only add channel if it is not already added

[schlichtanders]

Currently CondaPkg.add_channel(mychannel) is not idempotent, i.e. one needs to be cautious, not to execute it twice

CondaPkg.jl/src/deps.jl

Lines 248 to 251 in 55a039b

	function add!(toml, channel::ChannelSpec)
	channels = get!(Vector{Any}, toml, "channels")
	push!(channels, channel.name)
	end

It would be great if a little check could be added if a similar ChannelSpec already exists