fix: deduplicate peer deps

close #5373

.changeset/two-cobras-melt.md

@@ -0,0 +1,6 @@
+---
+"@pnpm/resolve-dependencies": patch
+"pnpm": patch
+---
+
+Deduplicate peer dependencies when automatically installing them [#5373](https://github.com/pnpm/pnpm/issues/5373).

packages/core/test/install/autoInstallPeers.ts

@@ -148,6 +148,23 @@ test('don\'t install the same missing peer dependency twice', async () => {
   ].sort())
 })
 
+// Covers https://github.com/pnpm/pnpm/issues/5373
+test('prefer the peer dependency version already used in the root', async () => {
+  await addDistTag({ package: '@pnpm/y', version: '2.0.0', distTag: 'latest' })
+  const project = prepareEmpty()
+  await install({
+    peerDependencies: {
+      '@pnpm.e2e/has-y-peer': '1.0.0',
+      '@pnpm/y': '^1.0.0',
+    },
+  }, await testDefaults({ autoInstallPeers: true }))
+  const lockfile = await project.readLockfile()
+  expect(Object.keys(lockfile.packages).sort()).toStrictEqual([
+    '/@pnpm/y/1.0.0',
+    '/@pnpm.e2e/has-y-peer/1.0.0_@pnpm+y@1.0.0',
+  ].sort())
+})
+
 test('automatically install root peer dependencies', async () => {
   const project = prepareEmpty()
 

packages/resolve-dependencies/src/resolveDependencies.ts

@@ -387,10 +387,10 @@ async function resolveDependenciesOfImporters (
   }
   const pkgAddressesByImportersWithoutPeers = await Promise.all(zipWith(async (importer, { pkgAddresses, postponedResolutionsQueue, postponedPeersResolutionQueue }) => {
     const newPreferredVersions = { ...importer.preferredVersions }
-    const newParentPkgAliases = { ...importer.parentPkgAliases }
+    const currentParentPkgAliases = { ...importer.parentPkgAliases }
     for (const pkgAddress of pkgAddresses) {
-      if (newParentPkgAliases[pkgAddress.alias] !== true) {
-        newParentPkgAliases[pkgAddress.alias] = pkgAddress
+      if (currentParentPkgAliases[pkgAddress.alias] !== true) {
+        currentParentPkgAliases[pkgAddress.alias] = pkgAddress
       }
       if (pkgAddress.updated) {
         ctx.updatedSet.add(pkgAddress.alias)
@@ -402,6 +402,7 @@ async function resolveDependenciesOfImporters (
       }
       newPreferredVersions[resolvedPackage.name][resolvedPackage.version] = 'version'
     }
+    const newParentPkgAliases = { ...importer.parentPkgAliases, ...currentParentPkgAliases }
     const postponedResolutionOpts = {
       preferredVersions: newPreferredVersions,
       parentPkgAliases: newParentPkgAliases,
@@ -420,17 +421,30 @@ async function resolveDependenciesOfImporters (
     const postponedPeersResolution = await Promise.all(
       postponedPeersResolutionQueue.map((postponedMissingPeers) => postponedMissingPeers(postponedResolutionOpts.parentPkgAliases))
     )
+    const resolvedPeers = [...childrenResults, ...postponedPeersResolution].reduce((acc, { resolvedPeers }) => Object.assign(acc, resolvedPeers), {})
     const allMissingPeers = mergePkgsDeps(
       [
-        ...pkgAddresses,
+        ...pkgAddresses.map((pkgAddress) => ({
+          ...pkgAddress,
+          missingPeers: fromPairs(
+            Object.entries(pkgAddress.missingPeers || {})
+              .filter(([peerName]) => {
+                if (!currentParentPkgAliases[peerName]) return true
+                if (currentParentPkgAliases[peerName] !== true) {
+                  resolvedPeers[peerName] = currentParentPkgAliases[peerName]
+                }
+                return false
+              })
+          ),
+        })),
         ...childrenResults,
         ...postponedPeersResolution,
       ].map(({ missingPeers }) => missingPeers).filter(Boolean)
     )
     return {
       missingPeers: allMissingPeers,
       pkgAddresses,
-      resolvedPeers: [...childrenResults, ...postponedPeersResolution].reduce((acc, { resolvedPeers }) => Object.assign(acc, resolvedPeers), {}),
+      resolvedPeers,
     }
   }, importers, resolveResults))
   return {