Users have reported facing this issue:

Duplicated timeseries in CollectorRegistry

The proposal to fix it is to change arq-prometheus interfaces and instanciate it outside the startup function and receive the ctx on start.

Example:

arq_prometheus = ArqPrometheusMetrics(delay=delay, enable_webserver=True)

async def startup(ctx):
    ctx["session"] = ClientSession()
    ctx["arq_prometheus"] = await arq_prometheus.start(ctx)

As of now I don't have the means to reproduce the error, so if you face it, feel free to submit a PR fixing. Maybe the suggested implementation works.