OpenGL Abnormal blending result

Question

Mesh (vertexBuffer and indexBuffer):

Several 2D triangles overlapped with each other. The vertex color of every vertex is R:1, G:0, B:0, A:0.005(transparent red). Triangles are combined into one mesh.

Rendering code: (C# code, but it should be the same for another languages)

GL.Enable(GL.GL_BLEND);
GL.BlendEquation(GL.GL_FUNC_ADD_EXT);
GL.BlendFunc(GL.GL_SRC_ALPHA, GL.GL_ONE_MINUS_SRC_ALPHA);
GL.Disable(GL.GL_CULL_FACE);
GL.Disable(GL.GL_DEPTH_TEST);
GL.Disable(GL.GL_ALPHA_TEST);
GL.DepthFunc(GL.GL_NEVER);
GL.Enable(GL.GL_SCISSOR_TEST);

GL.ClearColor(1, 1, 1, 1);
GL.Clear(GL.GL_COLOR_BUFFER_BIT | GL.GL_DEPTH_BUFFER_BIT);
GL.Viewport(0, 0, width, height);
GLM.mat4 ortho_projection = GLM.glm.ortho(0.0f, width, height, 0.0f, -5.0f, 5.0f);

material.program.Bind();
material.program.SetUniformMatrix4("ProjMtx", ortho_projection.to_array());

GL.BindVertexArray(material.vaoHandle);
GL.BindBuffer(GL.GL_ARRAY_BUFFER, material.VboHandle);
GL.BufferData(GL.GL_ARRAY_BUFFER, vertexBuffer.Count*sizeof_Vertex, vertexBuffer.Pointer, GL.GL_STREAM_DRAW);
GL.BindBuffer(GL.GL_ELEMENT_ARRAY_BUFFER, material.elementsHandle);
GL.BufferData(GL.GL_ELEMENT_ARRAY_BUFFER, indexBuffer.Count *sizeof_Index, indexBuffer.Pointer, GL.GL_STREAM_DRAW);

GL.DrawElements(GL.GL_TRIANGLES, elemCount, GL.GL_UNSIGNED_INT, IntPtr.Zero);

vertex shader

#version 330
uniform mat4 ProjMtx;
in vec2 Position;
in vec2 UV;
in vec4 Color;
out vec2 Frag_UV;
out vec4 Frag_Color;
void main()
{
    Frag_UV = UV;
    Frag_Color = Color;
    gl_Position = ProjMtx * vec4(Position.xy,0,1);
}

fragment shader

#version 330
in vec2 Frag_UV;
in vec4 Frag_Color;
out vec4 Out_Color;
void main()
{
    Out_Color = Frag_Color;
}

I thought the overlapped triangles would be rendered like this:

But the actual rendering result is:

After each drawing and SwapBuffer call, the red value of the rendered polygon becomes more and more stronger.

Why? Shouldn't those transparent triangles be blended only once? I mean, as the time goes by, the rendered result should be static, not animated.

Please note that I Want overlapped pixels be rendered in a stronger red color. But I Don't Want the whole mesh(overlapped triangles) be rendered in a more red color frame by frame. Since the current bound frame buffer is cleared, then the blending works correctly, the rendered result shouldn't be more and more red IMO.

OpenGL Abnormal blending result

1 Answers