In a project, I've found a code like this:

class SomeClass
{
    private SomeType _someField;

    public SomeType SomeField
    {
        get { return _someField; }
        set { _someField = value; }
    }

    protected virtual void SomeMethod(/*...., */SomeType someVar)
    {
    }

    private void SomeAnotherMethod()
    {
        //.............
        SomeMethod(_someField);
        //.............
    }

};

How better to convince my teammates that this code is bad?

UPD At least this is unnecessary complication. Why pass member variable as method parameter, if you already has access to it? This is violation of encapsulation, as well.
Any other considerations?

In a project, I've found a code like this:

class SomeClass
{
    private SomeType _someField;

    public SomeType SomeField
    {
        get { return _someField; }
        set { _someField = value; }
    }

    protected virtual void SomeMethod(/*...., */SomeType someVar)
    {
    }

    private void SomeAnotherMethod()
    {
        //.............
        SomeMethod(_someField);
        //.............
    }

};

How do I convince my teammates that this is bad code?

I believe this is an unnecessary complication. Why pass a member variable as a method parameter if you already have access to it? This is also a violation of encapsulation.

Do you see any other issues with this code?

In a project, I've found a code like this:

class SomeClass
{
    private SomeType _someField;

    public SomeType SomeField
    {
        get { return _someField; }
        set { _someField = value; }
    }

    protected virtual void SomeMethod(/*...., */SomeType someVar)
    {
    }

    private void SomeAnotherMethod()
    {
        //.............
        SomeMethod(_someField);
        //.............
    }

};

How better to convince my teammates that this code is bad?

In a project, I've found a code like this:

class SomeClass
{
    private SomeType _someField;

    public SomeType SomeField
    {
        get { return _someField; }
        set { _someField = value; }
    }

    protected virtual void SomeMethod(/*...., */SomeType someVar)
    {
    }

    private void SomeAnotherMethod()
    {
        //.............
        SomeMethod(_someField);
        //.............
    }

};

How better to convince my teammates that this code is bad?

UPD At least this is unnecessary complication. Why pass member variable as method parameter, if you already has access to it? This is violation of encapsulation, as well.
Any other considerations?