The short answer is that you need to include each file exactly once. Including a file multiple times (once via ammend_props.jl and again via main.jl) creates completely different types and functions that happen to have the same names, which is never what you want. I posted a few relevant answers back on this older thread that might be helpful: Modules and namespaces - #5 by rdeits
In your case, you could delete include("agent_type.jl") from Ammend_Props and instead just do using ..AgentType: Causality (note the .. instead of . since you’d be referring to the AgentType module in the parent Main.