Based on how this markup is written (<br> does not close <span>, so "Text 3" nests within the span for "Text 2"), I think duplication is to be expected (i.e no bug there AFAICT.) The offset of (one of) the "Text 3" mask is of course incorrect though (and probably, at least indirectly, an effect of the markup structure.)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/96b637071ae263682098a5f0659b67b6a2b43049

commit 96b637071ae263682098a5f0659b67b6a2b43049
Author: Fredrik Söderquist <fs@opera.com>
Date: Fri Aug 31 15:54:45 2018

Handle non-image backgrounds in BoxModelObjectPainter::PaintTextClipMask

When we paint a background color for a line, we don't adjust the paint
offset based on the logical offset on the "virtual line", so we mustn't
try to undo the adjustment in BoxModelObjectPainter::PaintTextClipMask
when we're generating the mask for "(-webkit-)background-clip: text".

Check the |object_has_multiple_boxes| and don't undo the adjustment if
it is false.

Bug:  879292 
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I9ec9c5cdfeb675803e54407f2e98e39b2dc1bc6b
Reviewed-on: https://chromium-review.googlesource.com/1199343
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#588038}
[add] https://crrev.com/96b637071ae263682098a5f0659b67b6a2b43049/third_party/WebKit/LayoutTests/fast/backgrounds/background-clip-text-multiline-linebreak-expected.html
[add] https://crrev.com/96b637071ae263682098a5f0659b67b6a2b43049/third_party/WebKit/LayoutTests/fast/backgrounds/background-clip-text-multiline-linebreak.html
[modify] https://crrev.com/96b637071ae263682098a5f0659b67b6a2b43049/third_party/blink/renderer/core/paint/box_model_object_painter.cc

Able to reproduce the issue on reported chrome version build without fix #68.0.3440.106 using Ubuntu 17.10 as per comment #0.

Verified fix on Ubuntu 17.10 and Mac OS 10.13.6, but could not verify on Windows as latest chrome build is not available on Windows OS.
Attaching screenshots for reference.
Observed all the text appears in purple.

Thanks.!

Deleted: 879292_fixed.png 125 KB

	879292_fixed.png 125 KB View Download

Deleted: 879292_without fix.png 140 KB

	879292_without fix.png 140 KB View Download