Oh My Zsh native plugin loader vs zplug: which should own plugin sourcing when both are configured?

I run Oh My Zsh with a handful of plugins listed in the plugins array in .zshrc, and I am evaluating zplug to manage a few additional third-party plugins that are not part of the Oh My Zsh repository. The boundary I cannot resolve is who owns sourcing when the same plugin could be loaded by either side.

My understanding is that Oh My Zsh sources plugins strictly in array order, with ~/.oh-my-zsh/custom/plugins overriding same-named built-ins, and that it has no dependency resolution or deduplication of its own. If zplug also sources a plugin that Oh My Zsh already loaded, I expect duplicate sourcing, but I have found no official guidance on whether the native loader should defer to an external manager, or vice versa.

Assume a current Oh My Zsh release and zsh 5.8+. I can verify behavior with zsh -x or temporary echo statements in plugin files, but I want a design decision before testing.

1. Is there a documented or de facto convention for partitioning plugins between Oh My Zsh's plugins array and an external manager like zplug?
2. Does duplicate sourcing of the same plugin cause concrete problems beyond startup time, such as re-registered hooks or doubled PATH entries?
3. Should custom overrides in custom/plugins be migrated to the external manager to keep updates clean?