Attaching modifiers twice to a sample

[alexander-held]

Summary

When adding a modifier twice to a sample, only one instance is used. For normfactor modifiers this could in principle be something a user would want to do on purpose (as a simplified way to scale different samples by mu and sqrt(mu), see also #850). It might be useful to catch such specifications and warn users about the current behavior.

OS / Environment

n/a

Steps to Reproduce

import pyhf

spec = {
    "channels": [
        {
            "name": "SR",
            "samples": [
                {
                    "data": [25.0],
                    "modifiers": [
                        {"data": None, "name": "mu", "type": "normfactor"},
                    ],
                    "name": "Signal",
                },
            ],
        }
    ],
    "measurements": [
        {"config": {"parameters": [], "poi": "mu"}, "name": "minimal_example"}
    ],
    "observations": [{"data": [35.0], "name": "SR"}],
    "version": "1.0.0",
}


def fit(spec):
    ws = pyhf.Workspace(spec)
    model = ws.model()
    data = ws.data(model) + model.config.auxdata
    print(pyhf.infer.mle.fit(data, model))


# fit with one modifier
fit(spec)

# add a second POI
spec["channels"][0]["samples"][0]["modifiers"].append(
    {"data": None, "name": "mu", "type": "normfactor"}
)
fit(spec)

File Upload (optional)

No response

Expected Results

warning or correct application of two POIs on the sample

Actual Results

[1.40000145]
[1.40000145]
# second modifier is ignored

pyhf Version

pyhf, version 0.7.0

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