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>> always @(posedge clk)

Is there an option for bothedge clk?

It's not synthesizable, but you could say "always @(clk)" as well.

always @(posedge clk or negedge clk)

This is the correct syntax, though many FPGA architectures don't support double edge registers

True, I directly answered the syntax-related question, but talking about implementation this would be a bad description - be it FPGA or ASIC.

As the parent's-parent comment said in other words, think circuit first.