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
Last updated for 1.11.2
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.