The bug

Beacon beams and structure block outlines will render themselves twice per frame if the chunk sector that the block entity is in is in view.
This can be observed by the opacity of a beacon beam changing:

The decreased opacity when looking upward indicates there is only a single render.

As side note this bug was introduced in the fix to MC-68247

Affected blocks

Last updated for 1.11.2

• beacon
• structure_block

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

This happens because the method net.minecraft.client.renderer.RenderGlobal.renderEntities(Entity, ICamera, float) first renders all tile entities and then all "global" tile entities. These are in the set net.minecraft.client.renderer.RenderGlobal.setTileEntities because their overridden method net.minecraft.client.renderer.tileentity.TileEntitySpecialRenderer.isGlobalRenderer(T) returns true.