project.json dependencies and tools

why does some tools are in the dependencies section, and what is the meaning of "type": "build". And why there are duplicate package in dependencies and tools?

here is the default project.json in asp.net core mvc 1.0 with individual authentication

"dependencies": {"Microsoft.NETCore.App": {"version": "1.0.1","type": "platform"
    },"Microsoft.AspNetCore.Authentication.Cookies": "1.0.0","Microsoft.AspNetCore.Diagnostics": "1.0.0","Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore": "1.0.0","Microsoft.AspNetCore.Identity.EntityFrameworkCore": "1.0.0","Microsoft.AspNetCore.Mvc": "1.0.1","Microsoft.AspNetCore.Razor.Tools": {"version": "1.0.0-preview2-final","type": "build"
    },"Microsoft.AspNetCore.Routing": "1.0.1","Microsoft.AspNetCore.Server.IISIntegration": "1.0.0","Microsoft.AspNetCore.Server.Kestrel": "1.0.1","Microsoft.AspNetCore.StaticFiles": "1.0.0","Microsoft.EntityFrameworkCore.SqlServer": "1.0.1","Microsoft.EntityFrameworkCore.SqlServer.Design": {"version": "1.0.1","type": "build"
    },"Microsoft.EntityFrameworkCore.Tools": {"version": "1.0.0-preview2-final","type": "build"
    },"Microsoft.Extensions.Configuration.EnvironmentVariables": "1.0.0","Microsoft.Extensions.Configuration.Json": "1.0.0","Microsoft.Extensions.Configuration.UserSecrets": "1.0.0","Microsoft.Extensions.Logging": "1.0.0","Microsoft.Extensions.Logging.Console": "1.0.0","Microsoft.Extensions.Logging.Debug": "1.0.0","Microsoft.Extensions.Options.ConfigurationExtensions": "1.0.0","Microsoft.VisualStudio.Web.BrowserLink.Loader": "14.0.0","Microsoft.VisualStudio.Web.CodeGeneration.Tools": {"version": "1.0.0-preview2-final","type": "build"
    },"Microsoft.VisualStudio.Web.CodeGenerators.Mvc": {"version": "1.0.0-preview2-final","type": "build"
    }
  },"tools": {"BundlerMinifier.Core": "2.0.238","Microsoft.AspNetCore.Razor.Tools": "1.0.0-preview2-final","Microsoft.AspNetCore.Server.IISIntegration.Tools": "1.0.0-preview2-final","Microsoft.EntityFrameworkCore.Tools": "1.0.0-preview2-final","Microsoft.Extensions.SecretManager.Tools": "1.0.0-preview2-final","Microsoft.VisualStudio.Web.CodeGeneration.Tools": {"version": "1.0.0-preview2-final","imports": ["portable-net45+win8"
      ]
    }
  }