pypa/pip · error · InstallationError

Double requirement given

Error message

Double requirement given: {install_req} (already in {existing_req}, name={install_req.name!r})

What it means

The legacy resolver detects two non-constraint requirements with the same name but different specifiers (resolver.py:254-267: has_conflicting_requirement checks parent_req_name is None, existing_req not constraint, same extras, differing specifiers). It raises InstallationError 'Double requirement' because pip cannot satisfy two contradictory version pins for one package.

Solutions

1. Reconcile the two specifiers into a single combined specifier (e.g. pkg>=1,<2).
2. Move the version pin into a constraints file (-c) and reference the package once in requirements.
3. Remove the duplicate/conflicting line.

Example fix

# before (requirements.txt)
pkg>=1
pkg<=2
# after
pkg>=1,<=2

# Detect duplicate/conflicting specifiers in a requirements file before install.
import re, collections
lines = [l.strip() for l in open("requirements.txt") if l.strip() and not l.startswith("#")]
names = collections.defaultdict(list)
for l in lines:
    m = re.match(r"([A-Za-z0-9_.-]+)", l)
    if m:
        names[m.group(1).lower()].append(l)
for pkg, specs in names.items():
    if len(specs) > 1 and len(set(specs)) > 1:
        print(f"CONFLICT: {pkg} appears as {specs}")

When it happens

Trigger: A requirements file (or CLI args) lists the same package twice with incompatible specifiers, e.g. `pkg>=1` and `pkg<1`.

Common situations: Duplicate lines in requirements.txt; a constraints file and a requirement line that disagree; merge conflicts leaving duplicate deps.

Related errors

• Cannot combine '--path' with '--user' or '--local'
• Cannot use '--only-dependencies' in combination with
• Could not satisfy constraints for
• [dependency-groups] resolution failed for
• Editable requirements are not allowed as constraints

AI-assisted analysis of pypa/pip@f399c37189 (2026-08-08). Data as JSON: /api/errors/e7a7d1d89fecf011. Report an issue: GitHub.

Appendix: source

Thrown at src/pip/_internal/resolution/legacy/resolver.py:264

        try:
            existing_req: InstallRequirement | None = requirement_set.get_requirement(
                install_req.name
            )
        except KeyError:
            existing_req = None

        has_conflicting_requirement = (
            parent_req_name is None
            and existing_req
            and not existing_req.constraint
            and existing_req.extras == install_req.extras
            and existing_req.req
            and install_req.req
            and existing_req.req.specifier != install_req.req.specifier
        )
        if has_conflicting_requirement:
            raise InstallationError(
                f"Double requirement given: {install_req} "
                f"(already in {existing_req}, name={install_req.name!r})"
            )

        # When no existing requirement exists, add the requirement as a
        # dependency and it will be scanned again after.
        if not existing_req:
            requirement_set.add_named_requirement(install_req)
            # We'd want to rescan this requirement later
            return [install_req], install_req

        # Assume there's no need to scan, and that we've already
        # encountered this for scanning.
        if install_req.constraint or not existing_req.constraint:
            return [], existing_req

        does_not_satisfy_constraint = install_req.link and not (
            existing_req.link and install_req.link.path == existing_req.link.path

View on GitHub (pinned to f399c37189)