vs for mac xamarin.forms 将pcl转换为netstandard后报错
报错内容：
Xamarin.Forms targets have been imported multiple times. Please check your project file and remove the duplicate import(s). (XF001)
解决方案：
经过一番谷歌后，找到解决办法。
原文：
If you converted the PCL project to .NET Standard via the project options the project will be using a project.json. NuGet will generate two files that also import the Xamarin.Forms.props and Xamarin.Forms.targets: projectname.nuget.props and projectname.nuget.targets. If your project file that uses the project.json file also imports the Xamarin.Forms.targets then that may be the double import.
I just reproduced that problem myself with VS Mac 7.3.3. I created a Xamarin.Forms Class Library (Multiplatform - Library) from the project template, then went into project options Build - General and converted the PCL to .NET Standard. Then tried to build the project. I see a build error Xamarin.Forms.2.5.0.121934/build/portable-win+net45+wp80+win81+wpa81/Xamarin.Forms.targets(3,3): Error XF001: Xamarin.Forms targets have been imported multiple times. Please check your project file and remove the duplicate import(s). (XF001). To fix this I edited the .csproj and removed the Xamarin.Forms.targets import from the file:
<Import Project="..\packages\Xamarin.Forms.2.5.0.121934\build\portable-win+net45+wp80+win81+wpa81\Xamarin.Forms.targets" Condition="Exists(‘..\packages\Xamarin.Forms.2.5.0.121934\build\portable-win+net45+wp80+win81+wpa81\Xamarin.Forms.targets‘)" />
翻译过来就是把 .csproj 中的<Import Project="..\packages\Xamarin.Forms.2.5.0.121934\build\portable-win+net45+wp80+win81+wpa81\Xamarin.Forms.targets" Condition="Exists(‘..\packages\Xamarin.Forms.2.5.0.121934\build\portable-win+net45+wp80+win81+wpa81\Xamarin.Forms.targets‘)" />删掉后重新打开就行了。
