Seeing Bing twice in a Zi Wei Dou Shu chart is not, by itself, an error. There are twelve palace positions but only ten heavenly stems. A sequence that continues through all twelve positions must repeat two stems.
Follow the sequence from Yin
Under a common placement rule, a Jia or Ji birth year starts the Yin position with Bing. The sequence then continues through the earthly branches. After reaching Yi at Hai, it continues with Bing at Zi and Ding at Chou.
Bing-Yin and Bing-Zi therefore share a stem but remain different positions. Nothing has been copied accidentally. Follow the earthly-branch sequence, rather than substituting the order of palace names such as Life, Siblings and Spouse.
Matching stems do not merge two life areas
In palace-stem transformation methods, two matching stems select the same transformation assignments. Their starting palaces still differ. A reading beginning from the Life Palace asks a different question from one beginning with the Wealth Palace.
Repeated stems also cannot establish that an event will happen twice. They are part of the chart's construction, not a count of marriages or career changes.
To check a chart, note the birth-year stem and the stem-branch label at Yin, then follow all twelve positions. Verify that sequence before interpreting the repeated labels.