Multiple precompiled assemblies errors after importing a package

First check: the file name in the error

Search the Assets folder for the DLL name from the message. Usually the same file appears under Assets/Plugins/ and in the package cache. Bulk-importing sample assets is the classic trigger.

Keep exactly one copy

The rule is simple: keep the copy managed by the Package Manager (UPM) and delete the one under Assets, or the other way around. If game code references the copy's namespace, move those references first.

// Example search in the file explorer
Assets/Plugins/OldSDK.dll      <- candidate to delete
Library/PackageCache/.../NewSDK.dll  <- keep

Reimport to clean the cache

If the error survives the deletion, delete the Library folder and reopen the project for a full reimport; the cache can keep referencing the old DLL.

Expected result and next step

A clean compile means done. When more DLL conflicts chain up, repeat the same procedure, and use asmdefs to narrow import scopes if references break. Build-stage conflicts are covered by the Android Gradle guide.

Error: Multiple precompiled assemblies with the same name
       NewSDK.dll (Assets/Plugins and PackageCache)